It's always a tough situation to be the step parent and in your case girlfriend of the parent. This means that his relationship with his child comes first. I think you must be a great girlfriend to go ahead and give up your bed for your boyfriend to sleep with his son. In some countries, family beds are the norm. In the US, this is not as typical and especially for older children. I understand your concern and I wouldn't want to sleep with my significant other's child of that age either.
So here is the big question, how does your boyfriend feel about it? Is he comfortable with it and okay with the situation? You can talk to him and share your concerns but HE has to be the one to come to the conclusion that he needs to help his son with some independence. Sometimes making an alternate place to sleep really attractive helps and this includes new cool linens (with something he likes on them), decorating the space to his liking, a cool night light,etc. Letting him have as much 'dad' time as possible before bed time is also helpful. That might be a good time for you to find other things to do. Kids like to feel they are most important to their parent--------- and they should be. So, without having anything against you at all, he'll need some time with dad on his own.
But DAD is the one who has all discussions with him and works on the problem. You are the boy's friend only. You can work on things with dad behind the scense but never let his son think you want to change things or he'll come up with a reason to be mad at you and bond less. good luck
In your shoes, I'd suggest to the dad that he work with his son on ways to ease the son's insecurity or possibly loneliness, without being cruel about it. Kids who are used to sleeping with their parents like to sleep with their parents. But this boy is going to have to separate sometime. So, things to keep him from feeling lonely or rejected, and things to boost his interest in his own bed and own room, and things to boost his self-esteem, will all help in this process. I would be gentle and understanding about it. Especially if you are new on the scene, the child might be feeling particularly ignored or insecure.