It sounds like you care about him. That's great. It's also wonderful that he is trying out therapy.
Its important to not push him to reveal anything he doesn't want to reveal. You don't actually need to know anything about his past in order to be his friend. Especially since he now has a therapist, you can just be there for him in the present. Let him know you are available, but don't pressure him to talk about things. That can really backfire.
Friendships need to have more than one dimension. Let him know that you are interested in hearing from him but that you guys can do and talk about other things too. All you can do is make yourself available. After that, it has to be up to him. Make sure he knows that you still want to be his friend whether or not he ever talks about it again. Then he can decide. Hope that helps.
Like a spent gladiator
crawling in the colosseum dust
who can count on his remaining limbs
all the people he can trust.
Like the one who stands behind him
cheering him on
Estatic when he stands defiant,
wild with abandon when he's gone
just stay alive.
do whatever you need to.
you are worth it.