My SD14 just came out as bisexual. I'm also bi and knew at around age 11 that I was attracted to girls in a different way than just being friends. I think it's very possible (since I lived it) that a kid knows that young. That's just my experience and now SD's, but my daughter says that a lot of kids in her high school freshman class are gay/bi. They're all between 13-15...

One of my good friends is gay. He knew he was gay since before adolescence. It's biological. If this 13 year old boy is coming you and his father with this information, love him, support him and do watch for signs of bullying. Thirteen is a rough age for kids, even rougher now that he's chosen to come out.

Instead of posting on here, go talk to your friends that are gay to get advice. The last thing you want to do is alienate this kid. They can probably give you a lot better advice than those of us that aren't in his shoes.