Since he feels comfortable masturbating in front of you; I think I would try to discuss it with him. I don't know how many conversations you've had about any previous relationships but that might be enlightening too? What about the rest of his life? Does he have friends, siblings, good job, etc. that you know about/have met? It's hard to picture him at all in a "regular" context. Is he still a older teen, in his mid-20's, at school?