Co-worker keeps flirting with me and he has a girlfriend. What do I do?

Okay this is definitely a problem. If he has a girlfriend then I think she would want to know about this, I mean just put yourself in her place. Wouldn't you want to know if your boyfriend was doing this to some other girl? I do think that she should know but you do seem really sweet and might not want to hurt her and I agree with that so maybe just tell him to back off for a while until he can control himself and if he keeps trying then you need to tell her. Don't let her get hurt and don't let yourself get hurt by falling for him or be the bad guy by letting him continue to do this. Good luck to you.