If he wants to hang out with you and you don't want him to, by all means, tell him she has a boyfriend, tell him you guys have somewhere else to be, tell him you don't want to hang out with him. Be direct but don't be rude.
It is unfortunate that people lose their sense of social grace in convention space, and behaviors that can go ignored in daily interaction or don't have an opportunity to pop up/get bad suddenly get magnified because we're all crammed into a shared space for three days. Creepers (male AND female) who can't follow you home after spotting you at a mall can suddenly follow you around everywhere including to your safe sleeping space. I support the growing "Cosplay is Not Consent" movement, and any con that has enacted this policy should be a good place to feel comfortable to go to staff/security if you can't shake off someone who has lost their sense of personal boundaries for the weekend. But I do encourage people to at least make it clear to the creeper in question that they're not wanted. A lot of creepers don't think they're being creepy, they're not sitting in their hotel room plotting which girls they're going to skeeve all over or whatever. They genuinely may lack social skills or they may be completely forgetting everything they ever learned about how not to talk to the opposite sex. It doesn't make them any less creepy nor does it excuse their behavior, but it means they have a blind spot and need a big neon sign to let them know before they find themselves face to face with a beefy security guy.
Founder and President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com