Bwahahaha! I think you have to get to them young, while they're still in they're in their formative teenage years.

Well, that, and I got him through anime. Series like Fushigi Yuugi which is mainly an action/romance animated series, but still features a canonically gay character and a couple of guy with questionable sexuality.

Generally, I think it's like my interest in video games. I'm not really interested for myself, but I'll pay attention to them and discuss them with him when he's down, because it's something he enjoys. Ditto for him with slash. He wouldn't read it or write it, he wouldn't go looking for it, but he'll watch something with me and see the amusement factor. *g*