Pre-docs and the idea that you have to have pre-doc experience to apply for a good PhD is more of a US thing. It's something that is creeping in in the UK, but stand alone Pre-doc / Research Assistant jobs, as opposed to Research Assistants who are concurrently registered as Master's or PhD students, are still much more rare in UK economics.
In Economics, those that do exist tend to fall into two groups. Firstly, there are ones that are fairly explicitly conceived as additional preparation for PhD study in the UK or internationally (these are the ones that tend to be styled as Pre-docs) and which, as a result, tend to be very competitive and look for people who can demonstrate that they will be excellent PhD students in the future and compete for places on the most competitive PhD programmes (hence why the people he knows who are doing them were very impressive as students). Views in UK academia tend to be rather mixed on the desirability of having a system that includes Pre-docs and on the true benefits of these Pre-docs versus just going straight onto a PhD (whether a standalone PhD or a 1 / 2 + 2.5 / 3 / 3.5, etc. one).
Secondly, there are just Research Assistant jobs that aren't explicitly expected to lead to anything at all, they are just jobs, and often quite poorly paid ones. Someone might do these jobs because they want to get more research experience to help them apply for a PhD, but they aren't designed with this purpose in mind and as a result they can often involve really quite mundane work with little autonomy, which isn't ideal preparation for a PhD. Not all Research Assistant jobs are like this, for example, ones in Research Institutes tend to be better, but you have to look carefully at what the job will involve, what you will learn and what useful skills and experience you would gain from it that would be desirable for a PhD.
In terms of applying for actual Pre-docs, your DS's grades in his undergraduate and Master's degrees might be a bit of a problem in the same way they would if he applied for a PhD. Although I notice a tendency for some universities to advertise any old basic research job as a 'Pre-doc', the proper Pre-docs that are properly structured as useful bridging experiences into a PhD tend to be very competitive. The Economics Pre-doc industry in the UK has to a large extent been able to emerge because of the extraordinary competitiveness of entry (with funding) into the top 5 or so Economics PhD programmes in the UK and the top programmes internationally. As a result, people trying for these Pre-docs tend to be extremely well-qualified already - the kind of people who could fairly easily be accepted onto a funded PhD or Masters + PhD at a different university but who will settle only for the very top ones and are looking for anything they think will help them beat the very low odds. He will need to think very carefully about how he presents his industry experience in a way that demonstrates the value it would bring to a Pre-doc and to a PhD, as this is his only selling-point. If he was able to do a proper Pre-doc of the type that are considered quite prestigious, this would probably help him get onto a PhD and get funding at a lot of universities, provided he meets the other minimum entry requirements (which he won't always with the grades he has). Whether and how much it would make up for OK but not brilliant prior qualifications will vary by university/course, but of course having prestigious and competitive things on an application is a good thing.
For more general Research Assistant jobs (and the pretend Pre-doc ones), having industry experience in the same area would be seen as a positive for getting the job. The benefits of having had a Research Assistant job when applying for a PhD will depend on what the job involves and how your DS can show that it has prepared him for a PhD. In Economics, doing a PhD is generally a very individual experience in which you are expected to design and conduct your research with just some minimal input from your supervisors and you need to understand not just the mechanics of doing research, but also more advanced techniques and the theory behind what you are doing (which is why it is becoming more common for Economics PhDs to require an Economics MRes before starting the actual PhD and why funding for Economics PhDs is a bit different to other subjects), so just getting a job that involves doing basic research tasks wholly under the direction of someone else has some value, but not so much that it would offset questions about an applicant's academic suitability for PhD study. So it's important to look carefully at what the Research Assistant role involves and to avoid getting stuck in a job just because it's in a university when it won't offer the right opportunities.
Realistically, your DS having neither a first nor a distinction at Master's level is likely to rule him out of a lot of open competition Economics PhDs at good universities, either explicitly due to the stated entry requirements (if these are explicitly stated there tends to not be much leeway) or simply because there are plenty of people applying who do have firsts and distinctions, and only very extensive research experience, like years and years in a proper research career, would compensate. This is particularly so if he needs funding - in most cases it is far, far easier to get offered a place than it is to get funding, although it is not generally recommended to do a PhD without funding anyway. This wouldn't be the case everywhere, though. He would need to investigate different courses and make enquiries - there is not much point in him leaving what sounds like a good job now to pursue some distant future dream that might ultimately prove to be unobtainable, so it is a good idea to enquire now to get advice on the steps he could take. Keep in mind that some places will be deliberately vague in their advice.
His best chance now or in the future for a funded PhD would probably be PhDs that are looking for very specific knowledge or experience, so things like PhDs tied to specific research projects or ESRC Case / Strategic linked Studentships which have an external partner alongside the university. Because these types of PhD are developed for a specific purpose by the supervisors / partners prior to the student starting, they are only suitable for people with knowledge and experience in the very specific area the PhD has been designed to address and as a result usually attract fewer suitable applicants per role. Such PhDs also usually put greater emphasis on pre-existing research or work experience in the desired area and demonstrable research skills as the student needs to be able to basically hit the ground running in terms of engagement with the wider research team or external partners.
He could also think creatively about academic disciplines / departments in searching for opportunities. Economics is very competitive, but there are people doing what is essentially economics in other disciplines like business, social policy, interdisciplinary research and so on where competition can sometimes be less fierce for Research Assistant jobs and for PhD funding and entry requirements less strict, so there may be more credit given for post-degree research or work experience as well as for just having a 2:1 and a Master's. Finally, if he just wants to do economics research because he is very interested in a particular research area, rather than to have an academic career, there are opportunities in research organisations, consultancies, think tanks, the civil service, charities, etc., and while having a PhD can be helpful in getting and progressing in these jobs, it isn't always a requirement, particularly for economists with in-demand quants skills. People do also go on to do PhDs after working for a while in these jobs.