I know what you mean, but you can get rid of him in a few easy steps.
1. Arrange it so you're drinking with him this weekend.
2. Proceed to get pissed, or at least appear to be.
3. Let "slip" to him in a bravado fashion that you were playing away on a certain night during the week (make sure you specify the night and it is a night you were with your girlfriend).
4. Wait for him to tell your girlfriend that you have been unfaithful.
5. Wait for your girlfriend to realise that you were with her that evening and think that he is lying in order to break you up (if she doesn't make this connection then point it out to her.
6. Demand that she never speak to him ever again.
Hey presto he's a slimy lying stalker and you're the victim, everyone's a winner! (apart from him obviously.)