Most Helpful Guy
He doesn't sound mature at all, 'bonding' with a mother figure, sleeping with the ring. Be glad he's not wasting your time any longer after 3 1/2 years. One of you and maybe both were too young for a lifetime commitment!
Most Helpful Girl
I think that he is in that relationship for the wrong reasons, and that he genuinly misses you. But cheating on his girlfriend is not ok and definitely bad karma for any future relationship you might have.
Be honest and tell him that you still love him, but that you will never share him. If he wants to be yours he can, but HE has to make the decision. In the mean time, let him boil on that and live your life as if he were never coming back.