I really like him, I respect that he has a girlfriend, but he acts like he's flirtying with me?

It depends do you like him? If so ask him if he still goes out with his gf. If he says no, and u like him go for it. If he says yes, than confront him. Ask him why he always flirts with other girls. If he does not give a reasonable answer leav him alone and dnt give into his flirting. Good luck!!