The only thing that matters re this other gal is whether he's in an actual relationship with her. If he is, back off. If not ....
Also, since you think he's living with this gal, why not ask? If you can't ask him a basic question, I mean, it doesn't make a lot of sense to be in a relationship (or try to be in one) where you can't talk to the person.
And, if you think he's living with her, and he doesn't tell you, why would you want to get back with him?