My 16yo hasn't done them. Our view is this. She doesn't have a specific need for qualifications right now, and may never need them because she's interested in a hands-on practical career and university doesn't appeal to her at all. Even the most academic of her friends have not really enjoyed preparing the GCSE syllabus. Many teens, parents and tutors say that as far as learning the actual material goes, GCSEs are a distraction, and a more open-ended self-directed study allows better focus on the subject matter.
With home education, there's no reason to suppose it would be any better to do GCSEs at 16 rather than any other age, so she may as well wait to see which if any she does need and do them then. (This is sometimes called the "just-in-time" approach, versus the schools' "just-in-case" approach of cramming in a large number of GCSEs at 16 just in case they may be needed.) Probably this will result in her doing fewer GCSEs, at a later age, and with a high motivation.
To anyone who finds this idea dubious, I'd put this question: would you make your teen sit their driving test at 17 if they didn't want to? Would you worry that failing to get that qualification behind them at an early age dooms them to a restricted life?
However, I do appreciate that she's in a somewhat different situation from your son, Sammi. I am confident that she'll have no particular struggles with doing exams, and this makes us relaxed. We aren't saying "never"; we are saying "not now". It must be worrying for you if you feel that GCSEs may not ever be an option for your son. I think it is well worth looking into alternative qualifications in order to put your mind and his at rest. There are other ways to study which are not so exam-based. For example, some people do entry-level courses with the Open University. Some who want to go on to university might wait and then do a Foundation course. It's likely that un inability to cope with exams would preclude certain careers altogether, but perhaps he will be lucky and not want one of those careers anyway. For most careers, there are many possible routes to follow. Many people say they never needed their GCSEs at all.