We had similar issues concerning education. Our situation is a little different as we are planning a permanent move and my kids are UKC, but perhaps the info I've received can be helpful. We ended up having to send my son over a year early so he could start Year 9 and delaying the rest of the family for a year so my daughter could go into Sixth Form. We were flat out told by schools in the area we'll be living in that Year 9 is key and that would not accept a student, new to the UK school system, in Year 11. They said at best, they would put her in Year 10 and hope she could keep up, or at the worst, all the way back in Year 9 (totally not an option as far as my daughter was concerned and can't say I blame her!) So we delayed our return until Summer 2018.
My daughter (currently a sophomore in High School) has applied at two Sixth Form Colleges: they both asked for transcripts, course descriptions and samples of her math and English work. They both stated that she must take math and English GCSE's and would review the sample work to determine if she could handle taking the Level 3 BTEC she selected at the same time. If not, then the plan is to complete math, English and two more GCSE level classes in the first year then the Level 3 BTEC the second and third year. We still have the hurdle of residency - one college has a stipulation that you have to have been resident in the UK for the 3 years before start of term for tuition waiver. The other has no issue, as long as she a UKC she doesn't have to pay tuition. In your situation, you would definitely have to pay - no idea how much it is though.
For a temporary gig, the move may not be the best thing for your son. Not because of the quality of education mind you. You'd end up paying tuition whether he went to a private (public in the UK) or Sixth Form. I'm sure he'd get a quality experience in a UK school, my concern would be how that would translate when you return to the States because the two systems do not compare apples to apples.