You have a window open for him, to get an open
Very true. My wife would lay the guilt trip on my son for other things, like Scouts. The year before last, I took him up for a small plane ride for his B-day. When we were getting ready to go to the airport, she said something like "you were planning this, weren't you." With a smirk, I said, "yep," and off we went.
Since then, I've shown him some of the news broadcasts about child abuse among the JWs. I've also explained to him how lawsuits work (the basics), plus I've gotten him thinking about the Kingdom Hall finances, and why the new Kingdom Hall hasn't been built yet in our community. He quickly put 2 and 2 together and postulated that maybe all those lawsuits were to blame! He's just 11, but he seems to be 'getting it.' Thankfully, our extended families are not JW, and I've never been one myself.
Just be your boy's best bud, unconditionally. After a while, he'll see the difference between you and mommy. He might start asking, "how come mommy is like this, or that." No need to attack her personally, but you can attack the cult, and with some research, he should eventually 'get it.'