Apparently he self-identifies are straight since he's dating a female (most males dating females are straight). He could be bi, of course. But that's unlikely simply because most guys are not bi or gay statistically speaking.
You do have that kiss from when he was 10, but he was a young kid. He didn't even know what sex was then, so how much can we really superimpose sexuality onto that kiss. He liked you and was attracted to you, as a friend for sure, but to interject sexual desire into that seems a bit forced.
I had a similar friend. We were younger. It was the First through Third grade. His name was Tommy (last name reserved for confidentiality). We were very close as friends, and in hindsight I think I was sexually attracted to him, but at the time I didn't know what that meant. The things that really stick out in my mind as "not normal friendship behavior" is when during nap time we'd get out mats out, lay them together, and hold each other. Specifically, one time the teacher came over and said "stop that, get on your own mat." I remember she was not pleased with our behavior and I didn't understand what was wrong. I also remember going to his birthday party and the other kids were monopolizing his time, and I started crying in his bedroom. I remember his mother finding me and when Tommy found out he was upset that I was upset. He apologized (though he had done nothing wrong) and I got over it. By the time I got home, there was a message. We hit the red button and there was a song (?) from Tommy, and I swear I can remember it to this day, and it went: "I love you, I love you, I love you..." as the hook. It was objectively horrible, but I remember being so happy hearing it. My father who was standing behind me said to my mother who was sitting in the chair "he can't see him anymore." My mom immediately took up for me and said "that's ridiculous, they're just friends." My dad (who was surprisingly accepting of my sexuality only years later) stormed off to the bedroom and shut the door. I was allowed to see him, and it was as if my father adopted a "don't ask, don't tell" policy thereafter with respect to Tommy and I. I also remember kissing a few times, but it wasn't romantic, or that big a deal. We never did anything with genitals, other than grinding perhaps. Nothing of consequence. Then he moved away to Hawaii.
Here is how I connect to you. I started thinking of him for some reason when I was 27 or so, and I went ahead and looked him up on facebook. I knew his first and last name, and that he'd have some connection to Hawaii. After searching dozens of profiles, I found him! His profile was unsecure so I looked at his pics. The emotions I felt were so real. It was a combination of excitement, and fear, and sadness and happiness. I looked over his photos and quickly learned that he was straight (or at least straight identifying) and had a wife and young boy. He surfs everyday, and couldn't be happier. He's just as beautiful as when we were young, and in fact because of his tanned and bronze skin now he's even hotter. He has tattoos on his arm, which are tasteful. I knew not to do anything rash so I let it go a few weeks. I then realized nothing could come of harm from just saying hi, so I send an invite. He was thrilled to hear from me and we had several substantial back and forths. Curiously, he did say something like "wow you got hot," which I thought a little odd for a straight married man, but I didn't dwell on it. He seemed supportive of the fact that I'm gay, and he's invited to host me should I go to Hawaii (I would never take that up, it'd be too tempting and weird, I'd visit him, but I'd definitely have a hotel room and my b/f with me....I'm not looking to cheat on my b/f and commit adultery with a married man...I don't roll like that). So we'll see if anything comes of it, but it really was fulfilling connecting with him, if even to know he's okay and doing well.
I say, contact him, but DON'T EXPECT ANYTHING OTHER THAN THE SATISFACTION OF SAYING 'HI'. Don't say hi if you're tryin get some dick. Don't say hi to drop ab emotional bombshell in his life and remind him of kissing you. Don't say hi to otherwise hurt him. Say hi to simply say hi. If you can honestly say you don't have any other agenda, than connect. Otherwise, let bygones be bygones.