How will my son continue his studies abroad?

You'll be ruining his chances of going to uni if you do this OP. So I suggest you dont.

Or could he do his A levels at boarding school?

I think you need to find a way for him to finish his education in the UK. Can he stay with someone while he does his second year?

Would he take AS levels , as these were phased put in much of England. It would be really tricky to transfer partway through as A levels are two year courses.

^This. Even if he moved to an international school which offered A Levels in the subjects he was doing, there's no guarantee they'd be doing the same exam boards. I can't see how this would work for him tbh.

@LuckyMumOf1 agree fully with @notquitesoyoung
Don't move him at the end of year 12. It's madness.
State school - live with family / friends in year 13 (or one parent stay in the UK for another year ideally).
Private school - as suggested above.
He is old enough to fly independently to join you in the US for holidays.

Presuming current school is not a boarding school change schools for sixth form, day pupil in Y12 and boarding in Y13 if it's possible otherwise boarding for both years. You can't change schools half way through A levels - it won't be an option. IB is supposed to be slightly more portable but really depends on the individual and options available at both schools. Personally I wouldn't consider a US high school for a 17 turning 18 year old.

Do you absolutely have to do this? It's terrible timing for your son. My parents moved me between countries at that age and I was miserable. I'm 40 now and I still haven't really forgiven them. My sister, who is 18 months younger, had a much easier time fitting into a different school system.

Is there any way you could you move now (I mean, obviously not immediately, but asap) or wait until your son has finished school?

Have a look for an international school to finish off his studies.

I'm not sure kids can move at that stage! I'd look for somewhere he can stay/board for a year in the UK to complete his A levels.

It would be a waste to not complete his a levels and sit his exams when he's got so far with them. There really is not much of a cross over between the US and UK system once you get passed elementary school. An international school would be a good option if there is one close by to where you will be living.

Where we were living in the US, there were some European exchange students who came and spent their last year of school in the US and graduated with a high school diploma (but not British kids). I have heard people say that A levels are more similar to the first year of college in the US, so he will very likely be ahead academically - but there is usually a lot more choice in terms of subject options and electives if you're in a good school district. But many HS do also offer college credit courses which reduce the time students need to spend at Uni - assuming he is wanting to study in the US and not in the UK. If he does go to college in the US, unless you already have permanent residency, he will be considered an international student, I believe, and therefore it will be a lot of $$$$.

Would he take AS levels , as these were phased put in much of England. It would be really tricky to transfer partway through as A levels are two year courses.