Oh, I see! I was slightly confused by your question about GCSE subjects which are hard for HE kids to do. There are a few - drama and art among them - which are very very hard because there are controlled assessments. So people usually go for different subjects, or do those subjects but obtain a different qualification. The alternative qualifications may or may not be accepted by a college or uni as equivalents.
Otherwise though, your son has a broader choice than if he were at school. GCSE and IGCSE are completely equivalent, and then often there is a choice between several exam boards. You do have to identify an exam centre where he can sit them, which is a headache. Some charge far more than others, some only do certain exam boards, some will offer pretty much whatever you want if you shell out large sums of cash. Their policies on accepting private candidates can change from year to year. So that might dictate which exam board and subjects you choose. Some families are prepared to travel a long way and fork out for hotels to do the subjects they want or go to a good exam centre ("good" = reliable, accommodating, good communicators, comfortable environment - which may be essential for children with anxiety).
You may end up going to several different exam centres. Consider how much travelling you are willing and able to do in each exam season and check the published exam timetables before making a final decision. But if your son is like most HE kids, he'll probably spread the exams out over several years anyway, which gives flexibility.
Are you familiar with the HE exams wiki? he-exams.wikia.org/wiki/Subjects That is a good place to start. It has links to discussion groups.
You'll also want to be in touch with local HE parents, who can tell you about local exam centres, colleges, tutor groups etc.