Syllabuses are online at the exam boards. There are organisations that will sell you distance-learning courses. However, I'd be wary of them. The word on the street is that they are expensive, of variable quality and highly dependent on the tutor you are assigned.
If you want help with English, Catherine Mooney is a home educator who has put together a specific package for EHEers and covers all the exam boards. She gets rave reviews on the EHE exams email list. Mostly, people get the books and do it themselves. Again, the list can recommend the best books for each exam.
For music, higher grade exams seem to have more credibility than GCSE-level exams (and are certainly easier to organise). Anything over a bare pass at G6 practical is in the arena of UCAS points for A-level, not GCSE.
It might be a good idea to plan to do a couple of exams in November or January [I think the month has moved from N to J, but I'm not sure] to get them under your belt and remove some stress. Maths is usually a good one to start with - the answers are largely either right or wrong and the content is easily defined and accessed. Also good to start with something DS is good at, enjoys and is well through the course material.
Good idea to defer deciding what to do until after IGCSEs, once you have checked what your local colleges and sixth forms require.
Incidentally, as long as he is EHE before his 16th birthday, he qualifies for Child Benefit after as long as he is doing school-level study.