I think the situation is less bad than you think it is. You asked him out thinking he was straight, and it turns out he is gay. That's not the end of the world, and I'm sure he's not offended.
If you want to be friends, tell him. The conversation doesn't have to be any more complicated than: "I asked you out because I didn't know that you're gay. I was hoping that we could still be friends, though. You're a really nice guy and I think we would get along as friends." Then, invite him to something.
There you have it - he's now your friend. If you tell him you know he's gay but just want to be his friend, he's not going to think you're trying to convert him. He'll just be your friend. Assuming, of course, you're friendly and the two of you have things in common.
So just talk to him. It'll be just fine.