I think that you can not be in a relationship with him while he is in a relationship with someone else.
You need to stop fearing that you will lose him.
NEWSFLASH - You already did lose him.
So stop holding on to him. Stop meeting his needs. You can ask him to move out but have no EXPECTATIONS that he will or won't.
If you study infidelity you will find out that pushing him closer to the OW is exactly what you should do. It is more likely to break up their relationship by pushing them closer together. This is the case where you lose the battle to win the war.
As far as controlling what he does when he is with the OW, that is not going to be within YOUR control. You need to work on your fear.
