Don't be confused...
Here it is...if this was truely a poly thing, there would be mutual understanding and not restrictions. You and the other girl would both be his girlfriends and would share your times with him equally...even together sometimes. This is not to say that you would be shareing him sexually, but in other ways. You would both love him and both be loved by him in every way and you girls would both be friends and understand each other about him.
It sounds to me like he is with you for some extra on the side action but doesn't want to loose his regular thing too.
I would be careful of this.