Sydney, it is really, really important for you to first, work through what appears to be latent anger for your husband.
It's different if you were "piecing" and going to couples counseling. You aren't. Neither of you are in a position to have anything other than a friendship R at this time.
It sounds like he's working things out and is very confused about what he wants and if you get involved, you are cheating on your BF and enabling him to cheat on his GF.
He needs to go through this and he needs to work this out for himself, with the help of the therapist. If anything, let him know that intimate conversations with him are making you uncomfortable. Set boundaries with your conversations with him. Let him know that if he crosses those boundaries, you will hang up, or whatever consequences you set.
You need to know what you want (or what you are open to; and what you are prepared to risk). But he could just be working things out in his own head, confiding in you, and then you take the bait, leave your BF and find out that he just wanted to get things off his chest and doesn't want any new R with you.
