This is quite an interesting discussion and I'm a bit surprised by some posts and happy with others. You say you're only out to your family right? To plan on being more open about it and completely coming out of the closet? Or is this going to stay within your family and close relations? You see, I'm "openly gay", but not blatantly gay. I don't flaunt it, I don't fit the stereotype at all. Basically if someone asks, I'll tell them the truth without a worry, but I don't tell them myself without a reason. Oh, I'm different online though. If you've ever talked to me online, you know and I'll bring it up quite a bit. This is only because it feels quite wrong for me to not be able to freely discuss it offline and whatnot.
Anyways, I'm totally with the people who say to wait and drop hints to be honest. As much as I hate to see another gay guy being forced to keep shut about it, when for how long have we had keep silence about this? My god, high school was fucking hell for me. But, for the sake of causing the least amount of trouble, you should start up a friendship first. Worry more about whether this guy suits your tastes as a friend, rather than if he's going to accept you as being game, and a lot of the time, the two go hand in hand. Just pay attention to his personality, how open minded he is (talk about religion and other beliefs, you can even slide in a question related to homosexuality here) what's his poltiical affiliation, if any. You can just try to make a good friend out of him regardless.
Or, is being with someone who doesn't like gays going to make you uncomfortable? IF hiding it is going to hinder your hopes to branch out your social life or you feelings towards doing that. You should tell him now, but in a very decent manner. Everyone in this topics seems to assume that he's going to tell him "Hey I'm gay! What do you think?". Be nice about it, don't make it your main point in your mail/discussion/whatever. Discuss your life first and bring it up later or as a way to reenforce a point. There are ways to mention it without screaming it in someone's ears, be smart about it, play it safe. Basically, if you're going to have a problem with a homophobic person, than telling him now is the best thing for YOU to do so you can get another roommate because then it would be your problem as well.
Just think about how you'd feel in each situation and go with what sounds best. No one here is going to have a certain answer for you to go about without fully understanding either person. We're only able to guess at what kind of guy this is and how he's going to handle things. Hell, he could even be gay himself like others mentioned and there wouldn't be a problem, or would there be?