High school relationship problems! please help?

Recently my friends and I have been hanging around a lot with some guys. There is one guy that I get along with really well. We are very similar and always have a good time... one problem: He has a girlfriend. It's a long distance relationship of 2 years and she lives out of state. They have met like 3 or 4 times. The question is, what am I supposed to do? I know that we would get along great but I don't want him to think I'm trying to break him and his girlfriend up. I also want to be respectful towards their relationship, if I was his girlfriend I wouldn't want some girl trying to get with my boyfriend because there is obviously nothing she can really do about it. Am I supposed to tell him or suppress my feelings? Or should I gradually flirt with him?