I really like a guy, he has no idea and might be getting with someone else.. what do i do?

Last summer i started to really like this guy, but one of my friends at the time really liked him, and everyone knew she liked him, so i never told anyone.. i bumped into him a couple of weeks ago and he came over and gave me a hug and said hello, i see him about quite often and i'm really starting toi have feelings for him again. He's one of the nicest guys ive ever met. I've heard he's kinda getting with this other girl who i've heard nothing but bad things about and tends to sleep with the guy and then dump them.. so i dont want him to get hurt, but im not sure on whether to say anything or not :/ i'm so stuck and confused.. please please help me!