Moving from US High School to UK Secondary

Yes but the fact that maths and English are not dropped with IB renders the GCSEs redundant.

My friend’s DS moved to the U.K. at 16 a few years ago, after being at an international school in mainland Europe, so MYP and no GCSEs, did IB (it was rarer then), now he’s graduated from a U.K. university and got a job right away. No one asked him for his GCSEs but then he does have a degree now. Maybe for non graduate entry level jobs they may get requested, but if the OP’s DS does the IB that will testify ability in English and maths (if passed in the iB).

One of my DDs had a friend from Switzerland in her A level class, and he had not done GCSEs. The school accepted whatever his qualifications were and allowed him to study A levels. This was a private 6th form, though, and he was an extremely talented musician.

There's an American school in surrey

With relatively few exceptions UK schools offer IB in lieu of A level, rather than throughout, so still sit gcses at 16. Those that might offer MYP and diploma and also cater for SEN are pretty limited.

You can go to a school that offers IB, no need for GCSEs then and IB continues with English and maths, up to an effectively higher level than GCSEs. IB is accepted by U.K. universities, plus universities abroad, including US, and covers 6 subjects. 3 subjects are studied at higher level, so A level standard or higher (depending on the subject) and 3 subjects are studied at standard level. Maths and your country’s language are obligatory, at either level. It’s much broader than A level and more aligned to other countries’ school certifications.

Depending on where you intend to live there are schools which follow the America route? Obviously they are private rather than state.

You might be able to access college? They often do 3 years - for certain courses? I think secondary state schools maybe slightly harder without GCSE?

I think you have 2 issues. Your child is unlikely to walk into a specialist school unless you have £££ to fund it yourself. Schools like the one your child is at, are few and far between. They are also expensive unless you have an EHCP and the local authority agrees to fund it.

The other is yes, it'll be extremely difficult to access A Level content from the US curriculum, unless your child is used to AP level work. Roughly speaking, the US high school diploma is equivalent to GCSEs. A Levels are very advanced. English universities expect a high level of knowledge and are 3 years, not 4. There's no spare year for gen ed classes like the US has.

Thinking longer term it will be trickier to get further in education or work in UK without English and Maths gcse passes.

UK, there are education departments in England, Scotland and Wales which have slight differences. Could you state the area you may be moving back to such as Manchester, London, Kent? You will get better suggestions that way. Also, private, state or both for applications?