There are thousands of children though who suffer poor mental health and have to cope in crowded 30 in a class classrooms. I really don't think either school or home ed are a cure for social or mental health difficulties, both can exacerbate them in different ways.
That said, I think by 13 upwards, the child should, if they have previously been home ed, have a choice to rejoin mainstream education if that's what they want.
Neither approach is a cure-all though for social issues and anxiety and mental health problems- teenagers are as nasty as they are supportive and nice, and going to a mainstream comp certainly isn't going to help your mental health necessarily, or social anxiety- I think posters both romanticise what ordinary school is like just as a lot of home edders demonize what school is like as well. It's just a place, with other people, and people have the same problems in both contexts.
I am not talking about extreme isolating situations or abusive controlling parents who HE.
I think there should be a HE register and checks made, both welfare and educational, but that's not a popular opinion in HE.
Is your son worried about re-entering more formal education later on, say for college or uni?