I think you already know the obvious that having the child in the bed with you is a hindrance to your relationship and in the long run can be less than ideal for the kid. The thing here is though, that I'm guessing, you're not married, you're not even a step father. So the advice given related to finding solutions is kind of useless. Why? because whether you like it or not, this is going to be your gf's choice and she has to do something about it. Now if she's asking you for help in this area, all doors are open to finding ways to get him into his own bed through the night.

So with that out of the way, first thing you need to do is communicate with your gf about the impact this is having on you and the relationship and everything related to that, you need to get her on your side of the court to deal with this. She obviously does not see it how you do yet and that's the first step. Get her to want to move the child out of the bed with her or it's always going to be a battle with the child as he'll always know he can bend his mother his way.

on that note, there are ways to do this and I've done this with my own children so... have hope.