Whats your take on this situation?

[Secretsbuild]

Alright well hi, I would like your feedback on this situation. Please nothing rude, no ads or anything such as, thanks.
About two years ago I started talking to this guy and we were instantly just kind of.. cool with each other. Then about a year into the friendship, after he'd told me he liked me twice, and also said the same thing to my sister, he started dating this chick. So i was like, okay whatever I'm not gonna be anyones mistress. So I backed off that, but he didn't. he told me he liked me a third time; at which point I lied and said I didnt like him because he wasnt my type. I wasnt going to lead on a boy whos in a relationship, and I wasnt going to tolerate a boy who told me he liked me and told my sister he liked her, all while in a relationship. then he calls my friend Kat and tells her that 'oh he likes me and thinks im really sweet and mature and that hes liked me since we started talking' and so on and so forth. Now.. Kat's the kind of person who.. immediatly called me up afterwards and told me what he said. And he knows shes that kind of person. Then last June, he started acting weird and wasnt telling me something - which was weird because he and I were always tight friends, always cool with telling the other whatever needed to be said (to a certain point.) So i called him and I'm like dude, whatsup what are you not telling me, and he got really nervous and by this point I'm just like -.- because I know whats coming, and he goes 'I really really ...REALLY like you.' and I, of course, said Dude, youve got a girlfriend. and he said Yeah, and now my feelings are really conflicted blah blah blah all this ****. And then as soon as he was finished talking I told him i had to go cause it was like midnight. Then he kept flirting and telling me he liked me over text, which made my sister mad because he had stopped texting her maybe like five-seven months ago but she's still hanging on.
Whats your take on the situation?
Please keep in mind I have been told by several people he is indeed a manwhore, which I can see. But hes my friend and I do like him, but I dont know how to handle it. then he moved and hes coming back up in January and we're gonna chill and if he tries anything I'm going to have to tell him to back the f u c k up. or if he says anything I'm going to have to tell him to stfu. I dont know. how do i handle it? hes leaving at the end of the year to be a marine, too. Help?

[TonyToneTone]

He's a player gurl!