MIB>> I was in a similar situation to your son, I was raised as a Witness from a very young age until I finally started cutting ties around 18 or 19. For a very long time, when it came to the the idea of religion in general and the Witnesses in particular it wasn't just anger I felt, it was outright rage. I felt abused, I looked back at all the pain and shame and confusion and the lost childhood that resulted from my witness upbringing and I hated. And sometimes it didn't really matter who, I just hated.
I talk a little more about why here, but ultimately the Witness upbringing, particularly the seperation and isolation is abuse - it's abuse in the most insidious form because the WTBTS convinces parents that what they're doing to their kids is right, it's good, and parents do it because they love their kids and want the best for them.
I can only really speak for myself, but I don't think that anger ever really goes away. More than a decade later, I refuse to see my mother who remains a Witness primarily because of the religion - I refuse to allow her the opportunity to poison my family as she poisoned hers. But time does give distance, and the opportunity for that anger, perhaps not to fade, but become something that we can come to terms with.
Having him spend some time with a therapist would be a good idea, but I do agree with some of the other comments - first, be prepared for him to reject the idea to begin with and second, it needs to be a therapist that has training or experience in working with religious cults and the psychological consequences. If he's unconvinced, then point him over here. Growing up in the Witnesses is a unique experience - it really is one of those cases of if you haven't been through it, you don't know what it's like. With that, I know I and many of us would be more than willing to talk to him, even if it's just about what he's been through. Compare battle scars as it were ;)
In any case, I'll keep my fingers crossed for him (and damnit, I'll enjoy it too, because even that is a defiance of the Witness "there's no such thing as luck" BS). Let us know how things turn out.