Hi, sotrue,
Glad you are here. He doesn't need your permission to get divorced, but to be honest, I don't see any point in standing in his way.
I had no "reason" to get divorced from my first husband--I simply didn't want to be married to him. It didn't feel right to me. Whether that was logical or "fair", it was how I felt. He, too, was quite hurt, but he got support from AA (he was sober when we got married and is still sober today) and from a therapist. Today, we remain good friends, and he is happily married to someone else.
There is no point in trying to hang on when the other person wants it to end, regardless of the reason. The sooner you get to a place of acceptance, the better off you will be as you start your new life. As we sometimes say around here, "Let go or be dragged."
Hugs, hope you will stick around, and perhaps check out Al-Anon.