My guess is that your boyfriend is caught in a dilemma. He wants to be with you. But he feels guilty about leaving a wife who has cancer. He may also not want to “look bad”, thinking that people will judge him for leaving a wife who is seriously ill. The way he resolves his dilemma is to try to have it both ways — He has a full and private life with you but presents a picture of a loving husband to his wife to the world. I suspect he hangs on to the item from his marriage to remind himself that he is still “married”, even while he has changed his entire life to be with you.
Arguing with him about “letting go” isn’t going to be helpful. He needs to find a way to both separate from his sick wife and feel like a decent guy. If he can’t figure it out on his own, I do suggest that a therapist can provide both practical suggestions and support.
Meanwhile, I suggest you be patient. Your bf isn’t acting “against” you. He has already proven how much he loves and values having you in his life. No. He is trying to navigate a very difficult internal dilemma. Try to find a way to give him some support.
I wish you well.