Economics is not the same as Finance.
A career in finance would not be out of the question for your DS. However, a career in particular types of finance (the most mathematical ones) would be more difficult for him to access because these specific careers do prefer graduates from the highest ranked universities (Cambridge, LSE, UCL, Warwick) simply because the highest ranked universities also have the most mathematical pure Economics courses and economics graduates of these universities do maths of a certain type and to a higher level (if they choose those options) than economics graduates from other universities. With AAB grades, these top ranked pure economics courses would not, as you note, be an option for your DS.
This is a cut and paste of something I wrote on another thread about different types of economics jobs:
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There are different types of economics jobs and different Economics degrees will equip you more or less for these jobs.
There are jobs that are heavily mathematical, for example in some areas of finance, insurance, pensions, plus more niche areas in transport, health, energy, etc., as well as some jobs in academia and consultancy. Big data and data mining, while it has generated more jobs in statistics, has also opened up demand for people who can do 'hard maths' in sectors where there previously wasn't much demand, the obvious example being advertising and communications. These kinds of jobs often involve day-to-day application of high maths and economic modelling in areas such as forecasting, planning, risk management and some policy-making. To do these jobs, you really need to be a very competent all-round mathematician and employers will not be looking for any old person with an economics degree. The kinds of courses that prepare people for these jobs are the ones that usually require A Level maths and prefer further maths as well.
Then there are jobs that don't really require you to be a high level mathematician, but they do require you to be a very competent statistician. These are jobs where you are still doing maths, but you are dealing more in applied maths and data interpretation. There are a lot of economics jobs that fall in to this category and it's a growth area generally, although there are a lot of not especially well-paid jobs in this area (compared to the average salaries of economics graduates). Some employers in this area would just want someone with an economics degree, but more likely, they would want to assure themselves that the person had some confidence and experience with maths, as evidenced through their having taken maths / stats-related economics modules and/or having A Level maths.
Finally, there are economics jobs that don't really involve a lot of doing maths. These jobs would include some areas of management and business functions more generally, some parts of investment banking, technical writing, some policy-related areas and other jobs where you are working predominantly at a strategic, rather than a technical, level and your job primarily requires you to interpret data, to understand it and to be able to communicate and apply that understanding. For these jobs, employers would just want an economics degree (or often just any degree) and economics degrees with a strong applied focus or those that allow you to focus on things like economic policy, history, social statistics and so on would be a good choice.
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In terms of courses, firstly be aware that Economics has become a very popular degree subject. This means that courses that ask for A*AA, for example, do not usually need to give much consideration to applicants whose predicted grades are a non-contextual AAB as they will already be needing to reject applicants who are predicted A stars because they get so many of them. Even courses that list AAB as their required non-contextual entry requirements will not necessarily make offers to everyone with AAB predicted grades. The more over-subscibed a course is, the more heavily they will weight the personal statement to differentiate between applicants who all have the necessary grades. It's fine to have some aspirational choices, there are, after all, no prizes for getting five offers. However, it is important to appreciate that for a subject like Economics at many RG universities there may be less leniency on grades than there might be for other subjects and to plan your application strategy accordingly, either including some safer options or aiming high and accepting that you may find yourself without any offers and needing to go through clearing with the limited choices that come with that. There are good Economics courses at some non-RG (and equivalent) universities, e.g., Essex and Surrey. I would personally be wary of BA Economics courses at lower ranked universities, i.e, ones ranked down in the mid-20s/30s and below, as usually that indicates that any maths taught on these courses will be pretty basic and low-level, providing little in the way of the desirable skills usually associated with Economics graduates.
In terms of content, most universities will have a first year compulsory core of macro, micro, some kind of maths or statistics, some kind of economic history/politics/philosophy/ideas module that is basically 'economics in the world'. There will be some courses with a slightly different core (and a few with a very different intro core) and the level and degree of abstraction will vary, with, as a generalisation, the higher ranked for Economics the university is the more theoretical in their approach and more concept and maths heavy their course will be, while lower ranked ones, and ones with a specific focus like business, will tend to be more focussed on practical, real-world application. There are exceptions to this, though. Some courses will also offer optional modules in the first year but others won't.
From the second year onwards, students will be able to take optional modules to focus on the branches of economics or skills that are most interesting or useful to them, so things like econometrics, development, behavioual, health, environmental or experimental econ, economic history, economic thought, public or other policy, trade, finance and banking and so on.
Better courses (imo) will continue at least in the second year to have a compulsory core of macro and micro because these are the fundamentals of economics and the building blocks upon which all economic thought is built, but it is increasingly common for universities not to have this post-first year macro and micro core because the students consider it boring and want to do fun and trendy modules instead. Two things that really seem to suprise students coming from A Levels are a) how much maths is required for most economics degrees and b) that there are actual fundamental principles in Economics that you need to learn - whatever you are interested in, whether it is Financial Economics or SuperFreakonomics, you need to know and understand the economic theories and concepts that underpin them so that you can then apply them to whatever interests you, and often that means putting in the hard, and sometimes boring, work before you get to do the cool stuff (this is a perpetual bugbear of Economics academics - if you can't summon up some minimal interest in the actual core knowlege and principles of the subject, please just study something else that you are actually interested in).
Some courses might also have other subjects in a second or subsequent year compulsory core, such as mathematical modules and research methods. You may also be able to do some modules offered by some other departments if you want to, although this sometimes works better in theory than in practice. When looking at optional modules, be careful not to fixate on a single module that you find particularly interesting, as there is no guarantee it will be available as staff leave, go on research leave and so on or it might be cancelled due to insufficient uptake, for example. Your DS should also consider if he might want to do a year in industry or study abroad and, if he does, investigate how that works at a practical level in different universities, e.g., Who is responsible for sourcing these opportunities? How, if at all, are they assessed?
I generally take the aspirations of 17 years olds to 'work in finance' with a pinch of salt (unless they already have some work experience or specific knowledge of finance) since it is so very, very, very common amongst prospective economics students and often what they actually mean is that they want a job that they perceive to be prestigious and well-paid (which is absolutely understandable and fair enough) and there are many jobs suitable for Economics graduates that 17 year olds have simply never heard of but that broadly meet those criteria without being in traditional finance. Nonetheless, if your DS has an actual interest in finance itself, he could consider looking at economics courses that are based in Business schools (generally economics courses will either be part of a broad social sciences faculty, particularly if they are very big courses in their own right, or a specific business faculty) as these courses will tend to offer more business and finance options, both within and outside economics, and some will orientate their learning specifically towards business, using business / finance examples when covering different economic concepts. Off the top of my head, from the Russell Group the business school-connected departments would include Birmingham, Cardiff, Exeter, King's, Leeds, Liverpool, Newcastle, and the ever-unusual Durham. There are probably others. QMUL economics is also very finance-based as a course. However, most pure Economics degrees will have sufficient options available for someone interested in finance to find enough interesting and relevant options.
As others have said, one thing to consider would be whether he would compromise on doing pure Economics in order to get into a higher-ranked university overall. Some (but by no means all) 'Economics and ...' courses will ask for lower entry grades, as will some 'Something Economics' courses, e.g., Industrial Economics. These courses may not offer the same grounding in pure Economics, which would impact on career prospects in certain areas of Economics, but a higher ranked university and a specialism / additional specialism may be more helpful in others. It is an incredibly high risk strategy to start a degree with the expectation that you will be able to transfer onto a pure Economics degree at the same university, particularly when entry to the pure Economics degree is very competitive, so I would advise against choosing a course with that hope in mind.