I saw you said about A-levels, I will say A-levels are so specialised, that you won't be able to teach him 2-4 A-levels well without capping hin at a certain level. By that I mean if you are at a 7 for example in maths as you said, you can't exactly teach him higher because your own ability is capped, that impacts his, so for A-levels if you were capped between B-D for a subject, he's also capped at that level. For A-levels, he will need a tutor, or something closer to formal schooling, perhaps through the ECHP you may be able to get a place in a special school provision etc if he chooses to do A-levels and not an apprenticeship or BTECs.
Are you sure he'll get his access arrangements? Online school depending on which one etc, doesn't necessarily always have to meet the same threshold as a formal exam, with JCQ etc and they may do it foe internal exams but it doesn't necessarily then transfer over. They sometimes need to do further testing, as it's not just you have x so you get extra time, or rest breaks etc. So ADHD on its own wouldn't necessarily mean he gets some arrangements or eveyrthing. As different people need different combos etc, so I'd check it's all gone through with JCQ and other bodies to be approved by them, if they do a spot check etc. He may get some but not all as well, so I'm guessing he has formal diagnosis for anxiety etc, so would likely get a smaller room with only a few people or a room with just him and the invigilator, but extra time, rest breaks etc, he may not get them all, so I'd get each one. Just because you want to make sure it's all definitely sorted and not leave it too late.
I would get him into clubs etc, brothers are great but he should still start to slowly get more sociable doing things he enjoys. So any sports or clubs, special interests etc. Also I'd start taking him to different places, like do 1h a week in the library, still the two of you but gentle controlled exposure over time will be good, as he grows in independence. Obviously at the right time but you don't just get ready for these things, your never fully ready, but slowly with you there is better than later.
Are his siblings older or younger? I'd still make sure they are okay, I know you likely do, and DS was in crisis but sounds like has improved a lot, but even if his siblings aren't that bad they still likely need lots of time and attention, and it must be so hard for you, but even spending 1-2h each with them doing an activity once a week, just 1-1 with both of them separately, doing a particular actviry they like etc, would be nice for them, they may not tell you as much etc, because they don't want to stress you out etc, but its good for them to get more of your time where you have energy, rather than when you are already wiped out. Not a criticism btw, you sound like you're very caring and DS definitely needed that but they have likely indirectly suffered from it, it's common when 1 sibling has more issues etc, it's nobody's fault but where you can try.