Summertime, I'm going to agree with ghostpost on this one.
He might already be out, but doesn't talk to his family because of their 'don't ask don't tell' policy.
For me, my parents know, and a couple of sprinklings of an aunt and maybe a few cousins, but as a whole, I have not told them. Mainly because I'm not close to that family, and my life has been built up without them. I don't think they need to know, because they aren't involved in my life.
If you really want to know, and care about him for more than just knowing if he's gay or not, get involved in his life, and show him you care. Get to know him. What he likes to do, what his passions are in life. Find out more about him than just who he chooses to love. I think by going this route, you may not even need to ask that question, the answer may present itself to you, without words.