FWBs are a bad idea. They never work out well. I doubt this arrangement will progress to anything meaningful. He won't be able to take you seriously because this relationship was not built on a serious foundation.
It is one thing to risk a friendship over a guy you actually have a relationship with and mutually care for. It is quite another to risk a friendship over a f*ck buddy. I think you should stop sleeping with this guy and hope your girl friend doesn't find out about it.
Relax... I'll need some information first. Just the basic facts - can you show me where it hurts?