First of all, congratulations on being a wonderful mom. If your son turns out to be gay, you are teaching him that you've got his back no matter what, that you'll be his rock in a hard, bigoted world, and that love will win out in the end. If he turns out not to be gay, you are modeling tolerance, compassion, and respect for difference.
My son expressed similar things at 5, but perhaps a little less decisively. He said he wanted to marry his (male) best friend, and not a woman, and was very interested to learn whether boys were allowed to marry boys. But it was never a strong interest of his or something he wanted to spend a lot of time on. So I think my experiences may be relevant but not quite identical.
The only suggestion I might have is, rather than being prescriptive -- saying "you shouldn't talk about these things" -- you might try expressing the whole thing in the third person -- "Some people still think it's wrong for boys to marry boys, but more and more of them are changing their minds. For the ones who think it's wrong, though, some people decide to be quiet and private about who they like, just so they don't give the meanies a chance to be mean." That way, on the chance that he turns out to be one of those guys who just can't be closeted, you're giving him an option, not only one possible course of action.
If you still think he'll be likely to be gay as he gets older, you might also see whether there's a Gay-Straight Alliance in his junior high or high school. PFLAG should also have some good resources. And for when he gets older (not yet), there are the It Gets Better videos.
Best wishes, and congratulations again for being the best of all possible moms.
Mira