Get him alone and in a low voice say, "dude, if you keep up the creeper routine, we're going to have to ask you to leave... don't stare at my GF anymore, got it"
Just kick him in the balls. He is doing a double insult. Eyeing up your GF and having eyes for another woman besides your mom.
You should be protecting your girlfriend. Your mother's boyfriend sounds like he might be dangerous. Don't jump to conclusions though. I'd actually talk to him about it and I'd say, "It looks like you were gawking at my girlfriend. I'd like you to know that I love her and I see that this is indignifying towards her. Stop this, or you won't have a relationship with me or her."
Call the Jeremy Kyle Show.........
Either suck it up and let him look, or pull him aside and set him straight. You're old enough not to need your mother to fight your battles. Just try to keep it somewhat friendly and non-physical.
Maybe you're jumping to conclusions. If it continues, tell mum so she is aware too. Once she confronts the bf, I doubt that he'll be much of a bother to you or your gf.
you should not bring your gf home.
Are you kidding? It ids his home. Why shouldn't he bring his girlfriend home? Where would you prefer he take her?