Hi OP.
First of all, try not to worry. If your son is doing the work set by his teachers, and you are helping as much as you can given your other responsibilities, then you are doing just fine. Nobody can ask either of you to do more than your best can they?
You are probably imagining everyone else doing so much more but I'm a teacher and can guarantee that they won't be - mostly they'll be doing as much as they can, just like you.
And remember that this is impacting every single Year 9 Pupil, so behind the scenes his teachers will be planning for the eventual return to school and ensuring that essential knowledge will still be taught.
In many ways, you are lucky that he is not in an exam year, or sitting exams next year, although contingencies will be made for those pupils too.
Socially, yes he may be missing his friends. It is a shame, but we all are. I think something like half the planet is or has been locked down. It is weeks out of his long life, and he will recover. Does he talk to friends in other ways?
Try to keep some perspective. If schools are back in September, he's missed a term of his school career, that's all.
And mostly try not to let your worries transfer to him. I'm sure you don't, but I see it so many times - parents fretting aloud and passing their concerns to their previously-oblivious kids. If you tell him to enjoy his unexpected term at home, and that he'll catch up just fine, he'll believe you.