Prom date disaster! hellP!!?

I don't know what to do. I can't leave my prom date because I would feel bad. A guy that is overweight and self conscious asked me if would go to prom with him, and I said sure but just as friends. I felt bad for him, he told me he's never had a real date before and besides it could be fun. Lately he's begun to get very annoying, pestering, and bothersome to me and my other friends and tends to stalk us all the time and follow us everywhere. He has already bought tickets, and talks about how excited he is. I can't just tell him its off, even though other guys have asked me to go with them that I really wanted to go with. My question is is it okay to dance with other guys for one dance at the prom that ask me, or my girlfriends? Or am I committed to this dude the whole night!!! My girlfriend said I should dance with him for a few dances and then just hang around and mingle with some other people at the dance. ?? At this point I am really wishing I was going with another guy who asked me...but this was after I had already told this guy yes. Plus I just felt bad...