People are being a bit harsh about the self starter thing. There are always lots of posters on Mn saying that sort of thing and clearly that’s the ideal, but most of the parents I know at very high performing London school are engaged in the uni application process, so don’t be put off, op.
either you or he need to do some intense internet research on courses and admissions requirements. Not having APs is a big complication (seems most odd - would have thought that would be a problem for US schools too) - I don’t think Oxbridge is feasible without, nor Inperial but check of course. Some places accept honours/dual enrolment courses together with ACT/SAT but it is very case by case - your son/ you writing as him will have to inquire with each admissions department. Is he going into his last year? If so, you need to work fast.
The UK system is massively more specialised than the US one, although they are probably a bit closer on the science side. If you are on a biology course, that is pretty much all you will study - no adding french lit 101 for fun. So he needs to know what he wants to do. Scotland is very worth considering as it does give some flexibility - at St Andrew’s you only spend 50% of your time on your billed subject and there is scope, by accumulating appropriate credits, to shift subjects for the honours years in 3 and 4. Also, if he hasn’t done AP, having the extra preparatory year would be a useful catch up. Further, there are loads and loads of Americans there, so may be an easier transition. Edinburgh not quite as flexible, but still much more so than English schools.
another angle he might want to explore is Natural Science courses as they don’t commit one to a single science strand upfront. The UCAS site has good course finder tools (stick in AAB for a levels for a suitable level) and lots of general info and for internationals.
Another big difference is style of learning and engagement. My friends’ description of US university sounds a lot more like school - heavy timetables of classes and lectures (“contact time”), continuous assessment / mid terms, parental involvement eg freshers’ parents’ weekends, ability to contact the school etc. UK is very different. Admittedly science degrees have a lot more contact time than humanities - circa 15-20 cf to 10 but it is still very far from a strict school-like schedule. If students stay in bed and don’t attend no one will know or notice for ages and whatever happens, they won’t be reporting back to parents. The student has to take control of their learning.
An advantage for overseas students is that their higher fees mean that offer rates and requirements are often lower than for U.K. students (except at Oxbridge and probably Imperial).
You and he should spend a few days diving into some suitable university websites to get a feel for courses, lifestyle and admission requirements. Off the top of my head, look at St Andrew’s, Edinburgh, UCL, KCL, Warwick, Durham, Bristol, Sheffield. That will give you a sense.
what is his SAT score?