I think you should give some more thought to the idea of home education, and learn more about what it is really like. It's a very liberating alternative which would allow your son to study whatever, whenever and however he likes. There are a lot of crazy things about the school system, but nowhere is its inflexibility so shocking as for a young person in your son's situation. Give it a miss.
Your son is looking forward to the excitement of being in a new country. It seems a shame to bring him here only to put his nose to the grindstone satisfying the demands of a rigid system. I can't see that he'd get much time to socialise at school during a time when all his classmates are knuckling down getting ready for exams and he himself is working even harder than they are to "catch up" with a "missed year" of education.
If you go with the home ed option, having a two year old is neither here nor there. A sixteen year old is unlikely to be getting taught by mum anyway, unless you have some specific background that would be helpful to him, in which case you could spend a few hours a week helping him. He'd be working under his own steam (good preparation for university), possibly with tutors or distance learning or a local home ed study group if he wants extra support.
He can choose to do IGCSE exams if he wants. If he does, he can do them at whatever pace is right for him, subject to availability of an exam centre at the time he wants to sit them. He can do as many or as few as he likes, in any order he likes.
He can do courses (part-time or full-time) at a local college. It used to be tricky to arrange funding for a Year 11 to do this, but from this September that is changing and colleges can fund young people in Y10 and Y11 directly. Colleges may or may not be flexible in accepting him without any GCSEs; experiences of home educated teens in attempting this vary. (Common wisdom is to approach subject tutors rather than admission tutors in the first instance, as the former are more likely to be flexible.)
If he's planning to return to an American university then he could go with an American curriculum. Or he could put together his own programme of study, choosing whatever is right for him in each of the subjects he wants to study; that "pick-and-mix" approach is very common among home educated teens here. If university is on the horizon then he'll want to keep an eye on what the universities would consider to be acceptable evidence of preparedness. I expect that doing ACTs or SATs would be a straightforward piece of the puzzle. Some universities are accustomed to receiving applications from young people who come from a range of backgrounds.
The Open University is very well-regarded and gives excellent support. It has been popular with academically-inclined home educated teens, though recent funding changes make the OU a less attractive option now than it once was.
Your lad could get out and about and learn that way. There are plenty of fantastic museums and historic sites on your doorstep and farther afield. Public transport is probably better than you are used to. My teenager and her friends roam around readily.
As for social opportunities, school isn't the only place to find friends. In your area there may or may not be a ready-made social network for home educated teens to plug into. If not, what about a part-time job? There are many interesting volunteer opportunities available to over-16s too. Or he can join a club or do sports.
You say that this move represents a great adventure to your son. Don't lose sight of that by pushing him into a restrictive learning environment when there are so many other options available.
www.educationotherwise.net/
www.nwilts-he.org.uk/he_exams_wiki/index.php/Main_Page
www.open.ac.uk/about/main/
vinspired.com/
edyourself.org/articles/WolfReport.php#emailtoparentapril26th