I could kick myself. There was a time when I had my wife against the ropes, in a way. She made conciliatory remarks..remarks that revealed doubts. I'd say at that point she was ready for some listening. I would say my biggest problem has been lack of follow-through. Well, developments since that time drove her very deeply into the org. So...don't ever let an opportunity go by!
Some times when they are ready, they will actually ask you for information (at one point my wife actually did this but I was so green...what did I do? stall..) You might be able to prime them to do this by finding ways to make them curious to find out what you're withholding from them about the org.
Now say they did start showing signs of wanting to learn a few things. WHAT would they be ready to hear? Steve Hassan mentiones you should find out a person's criteria for leaving "a" group. You can do this by bringing up hypothetical situations of people who are trapped in various group situations, through the use of various pressure tactics, and finding out what would be red flags for your husband about such a group if he was in that situation. What would be the dead giveaway for HIM that such and such group wasn't what they claimed to be? Would it be a legitimate reason to leave?
Perhaps a good, subtle discussions on this could arise from movies or historical documentaries of various groups.
Just throwing out some generalities to ponder.