He seems to be unable to shake the idea that they have "the truth". He doesn't live as a JW though, so he seems trapped in an intolerable state between thinking the JW are the way to go, but unable (and having no desire) to live as one.
This is, unfortunately, an incredibly common problem. There was a guy at my hall who had been disfellowshipped for decades and later returned. He would often talk about the agony of being out and knowing he wasn't living in conformity with "the truth." I believe a lot of the destructive behavior common in DF'd youths is about them beating themselves up over not being strong enough to live up to the "the truth."
I was in this state for a short while, something like a year and a half. What changed? I found this website. I read enough here to lose the fear of reading "apostate literature" and after a few books any lingering doubts about whether the witnesses had the truth disappeared. Don't get me wrong. The programming runs deep and even today, ten years later, I still find myself responding to some situations according to the programming, but I don't agonize over whether I made the right decision.
Without knowing your son, it seems the only thing that can help him get over this hurdle is for him to come to realize on his own that there is no truth to the religion. My best friend when I was a witness stopped going to meetings around the same time I did and I swear he's in your son's position. He won't admit it to me because he knows I'm 100% out, but it's evident by his demeanor and some of the choices he's made. That no-man's land of being 99% out but having those lingering doubts isn't a healthy place to be.
I wish you all the best.