First, I think it is great that you have the support of a therapist through all of this. It was a huge bombshell that your husband dropped on you, and I can only guess at how surprised you were by his wanting help.
It's great that he wants help, and great that he was willing to initiate it. My last marriage was destroyed by my ex-husband's addiction to opiates, so I am excited for you that your husband is addressing this.
I had a shock recently when I discovered that one of my dads had been in the hospital for a cancer prostate removal and had suffered some pretty bad complications. I hadn't even been told that he'd even had cancer beforehand; was only told after he had begun leaking fluid into his abdominal cavity, and then spoke to him a few days later when he came home.(he lives a ways away) So, while I was glad he was okay, I was also shocked that the illness had been diagnosed, hurt that I wasn't told sooner, and really frustrated as to what I could do to help or even how to talk with him about it. (He's not the most emotionally-available person.) Having the support of my husband through this really helped and if things had been worse, I likely would have called up the counselor I used to see and scheduled a session.
I think being authentic in how you feel you can help (your aversion to 12-step programs is noted!) is really important. Make sure your needs are not superseded by his desire to participate in group therapy. I am pretty sure you could go to a group session or two and quietly support him, and you could both agree that any aspects of the marriage that need to be addressed will be dealt with in private couples counseling sessions.
Of course there are a lot of feelings to deal with. Take them as they come. It's okay to be grateful that he wants help, and then to feel sad later that day that your family is dealing with this. Feel the feelings and then journal about them if you like, or talk with a very trusted person...whisper them on the wind and then--- let them pass.
Again, keep getting your own support. Know that this is just the beginning. Someone I love recently addressed some hard stuff in their marriage-- the first month was hellish for her, but a couple months out, she's feeling a lot better and glad that the marriage is far more stable than it was before when they weren't dealing with their 'stuff'.
Best wishes for you and your family.