I would not make an attempt to talk to the sponsor.
1. AA is anonymous. You're asking someone to out themselves. You're asking your EX to out his sponsor, too.
2. None of your business what your EX talks about with a sponsor.
3. He (your EX) is gonna do what he's gonna do, drinking-wise. It seems really intrusive to expect a non-paid non-professional peer support member to get in the middle of this.
Try looking at it from the sponsor's point of view. Your need to ferret out honesty is not his problem. It's not his circus and not his monkey.
If I was the sponsor, I would not agree to a meeting with a spouse for the purpose of letting an ex wife in on the private sessions. I don't think you can force something like this. Part of the success of the Steps is dependent on discretion on the part of meeting members and sponsors.
I think if I were going to AA for help but I knew someone was going to ask questions that could trip me up in court later, I would not be honest with a sponsor - and that will hurt recovery in the long-term.