What should I do, I like a guy that has a girlfriend?!?

Okay so last year this guy and I had a little bit of an attraction, he invited me to his party, I couldnt go, we didnt talk that much since then. Well at his party he got a girlfriend. Theyve been going out for 7 months? Now and they brake up constantly.

Well hes in my math class this year and I didnt like him at all the beginning of the year but he would flirt like crazy with me everyday, all day. People would yell at us during class, “just kiss already!” or “shut up love birds!” or silly things like that and everday he would ask me to go out with him, id say no (cause he had a girlfriend) but he would still declare to everyone I was his girlfriend.

Well now im reaaallly beginning to like him and it seems like he doesnt like me that much anymore and I dont know why?!?! I think he still likes me a little bit but hes still has the same girlfriend though but it doesnt seem like he likes her too much either.

And I just dont know what to do, sorry if this is all confusing but anyone who can help me, thank you so much!

• to add, his girlfriend and I are kind of friends I guess? She says hi to me and ill say hi back, we complement each other but thats it… I don’t know, I would kind of feel bad if I stole her boyfriend?