I know this sounds a little crazy but..?

there's this guy that I liked and he likes me to and he took me to this game which I thought was tottally fun and then he tries to come on to me and wants to touch me and all and then I find out from my firend that he has a girlfriend in which I know! What I don't get is why he would tell me he's with somebody but he's not. Thats bull**** and how can I let him to know back off because he's sweet and all but he makes me feel like a girl on the side that he'll just play like a videogame. I am still a little hurt but now I'm starting to get over it. But on Monday what can I wear to make him wish he didn't have a girlfriend but me instead, yet at the same time get it throught his head that he HAD his chance? And to really just back off. Because if he's sending me mixed messages, then I will too.