This thread seems to be a sort of bingo of MN tropes. Just because "everyone" says it, does not mean it is true.
- The one stretch, two reasonable and two safety, or some combination of these, is not necessarily the best approach when it comes to applying to oversubscribed courses. If there is an element of luck/chance involved in applications to Cambridge/Imperial etc, a better alternative can be applying to top ranking courses (hence increasing the odds of getting one) and then, if unsuccessful, taking a gap year or going into clearing. Courses demanding high grades will be able to start at a higher level and maintain a faster pace.
- Of course you can apply to Oxbridge and only add other choices just before the January deadline. It will not affect your chances at other Universities, and is hardly onerous, in terms of conflicting with school work. Indeed OP's son could add a couple of more competitive now and then add a potential insurance once they know what grades they need for their offers.
- TizerorFizz is new to this board. The merits of RG have been done to death. The strength of engineering departments depend heavily on how much the University or outside research has invested in a department. Engineering is an expensive degree, and some RG universities have focussed on, well , cheaper subjects. There are historic factors as well, so the old red brick unis are often good. Plus there are lots of different sorts of engineering. Surrey for automotive, for example. Does OPs son have any idea of what interests him? Larger departments can offer more options and more flexibility. What doors are opened career wise? What are the criteria for proceeding to the Masters? He should have a good look at the courses and the options available, and the research interests of academics.
In terms of where else to apply, one of DS's friends with five top grade A levels, went to Sheffield for effectively the same reasons. (Was not offered Cambridge, and did not want Imperial.) He had a great time and has done very well. Cambridge and Imperial can get very intense, and the short terms at Cambridge add to the pressure, whereas other places allow more time to explore other interests.
As well as Sheffield, Southampton and Manchester seem to be the main non-London Oxbridge fallbacks for DCs friends, though to be honest most head for Imperial. DD has just taken a year out from her medical degree and completed an intercalated degree in bio-medical engineering at Imperial, and was impressed. It was well organised, impressively so given Covid19 and students stuck all over the world: the IT worked, the summer school, courses, project work and exams were adjusted, and when there was an issue she got a response within the hour. The course, and to some extent fellow students, were intense, and she covered a huge amount of ground. She suspects this suited her better in her early twenties than it might have done at 18. There was a very good range of courses available, even in such a specialised area, and many of the people teaching her were engaged in internationally recognised research. She also enjoyed the sport, when allowed, and confirms her brothers earlier impression that London Universities have active societies and perhaps more going on on campus. She is very proud of having Imperial on her CV, and thinks it will help, especially if she chooses to head for research/academia.
(And for anyone reading with younger children, and to challenge another MN trope, DD found it useful to have taken more than 3 A levels. She had taken five, which probably helped in getting the place as her subjects demonstrated an interest in the maths/physics side of STEM, helped in giving her a bigger range of options and meant she was familiar with quite a lot of the early content.)