It is important for a teenager to feel a sense of accomplishment, and graduating will give him that. I think it is amazing that you and your husband have taken on the role of ensuring your brother is educated. Very kind and responsible of you
As for school... is he nervous because of social issues or acedemic issues? Is there a night school or alternate school he could attend?
Correspondence/Online courses are great -- for people with enough discipline. I would say, if he does online, try to come to an agreement that he will work on it for X hours per day, 5 days per week. That way, he doesnt push it off to the side and ignore it.
Good Luck - hope it works out!! And again, you are doing a great thing