Theres this guy, what do i do?

So heres the thing, im nto bad with guys at all, i find them easy but ive never been in a situation like this where i like this guy but he has a gf and he wants me too. i told him no because its not fair and a person like me does not like to share. yet he texts me, ims me and talks to me, we had a bit of a falling out but he is still here which amazed me, but i dont know what to do, i want to be with him but i dont wanna be on the side. I asked one of my best friends and he tols me that the guy i like seems immature and that i shud wait maybe hes not ready for commitment? it all makes sense, i just dont know if i shud wait or move on?????