Poly is more about love, respect and openess. So if S and A still love each other, I would consider it a poly situation.
I think most of the issues will be about communication. Ask him what bugs him. Let him know what bugs you. For the most pary it will be standard roommate issues. And there may be a lot of such issues.
On top of that, you can just try to reassure him that he is not trying to make S leave him.