I like a guy with a girlfriend??

Okay. I should probably start this by saying that I have no intention of causing him to cheat on her or to have them break up or anything like that, I just really like him. I'm a sophomore in high school by the way. So there's this guy that rides my bus and lives like 2 streets down from me. He's really really really REALLY cute and he's literally the nicest guy I've ever met and he is so genuinely nice to me he's my best friend. I have had the biggest crush on him since the day I met him. He comes over all the time and we ride out skateboards outside in my street and he always makes my day better. I've never liked anybody as much as I like him. The problem is, he has a girlfriend. They've been dating for nine months. He seems really happy and obviously I wouldn't want to mess with his happiness, I just like him so much. He called me pretty and said he loves hanging out with me and he compliments me a lot and he flirts with me almost constantly but I don't know if he's just a naturally flirty person or if he's actually flirting with me even though he has a girlfriend. I don't know what to do. Obviously I'm going to just be his friend. I don't want to mess anything up. I'm just asking for advice or similar stories. Thanks.