Can you just enjoy the relationship for what it is now? He may eventually find that monogamous partner to settle down with and yes it will hurt and yes it will be hard to get over, but why torture yourself with the what if's now? There is always the potential for a relationship to end. I think it's better to enjoy what you have. Be realistic that it might not be forever and be open to letting your relationship change as the situation requires. Since there isn't anyone else on the horizon at the moment just be and enjoy what you have.
Everything will be ok in the end. If it's not ok it's not the end.