I was asked to participate in a conference call with my xah when he was in an in-patient rehab out of state. I was really nervous etc. as well. I ultimately decided to do it. He had an excellent counselor that made things a lot easier. Some of the things the counselor asked were...how alcoholism affected me and our relationship, what would his recovery look like to me (what could I see that would indicate recovery), did I think there were other things he needed to work on (like anger issues etc.), the counselor outlined some things for me, etc.
The counselor talked to me ahead of time so I had some things written down because I don't do well in those situations if I don't write my thoughts out ahead of time.
Our conversation focused on our kids because we are already divorced and I have no desire to rekindle our relationship so just didn't go there. If I would have wanted that it would have been a great opportunity though. I imagine a lot depends on the skill of the counselor and program.
ETA: I would not hold back. Be brutally honest. The counselors will help him work through that. I think it is important to a) break down denial - I'm pretty sure that was most of the point of our conference call because like I said - we'd been divorced a year, and b) for him to get help in learning how to deal with his feelings, your feelings, etc. and he has tons of support there to learn some skills he can use when he gets out.