I think sometimes people get stuck blaming others. Personally I would have been thrilled of my JW mom left when I was 21. She's still in so he should be grateful you at least got out at some point.
Also, you know, I was pretty much "out" as a teen although my family was fully in. I don't mean to be harsh...but there are lots of cases of JWs who left at an early age or at least doubted and started doing what they wanted while still living at home and having witness parents. I wonder if his anger isn't somewhat misdirected and he's really angry at himself for not seeing things clearer and being brave enough to leave (with or without his parents approval).
I find very passive people who are people pleasers love to blame others for not having "allowed" them to do things. In actuality they've chosen to put other people's approval and feelings above their own needs but it takes alot of maturity and growth to admit that. It's quite a valuable lesson once you learn it.
Your son certainly could have chosen to go against the grain and left the JWs all by himself. There are plenty who have.
Maybe it's a phase, but eventually blaming others for the way your life is now gets really old.