ADD, I really feel for you, I cannot imagine how hurt you must be. I started a thread about being born in blurring the lines between JW issues and parental issues. This seems like that to me.
My take on your sons letter is that he is confused in his emotions. It may be that for a long time he has resented how hard you were on him but he could justify your being that way because of being a witness. Your apology to him while camping would also have been taken in the context of being a witness and would have built on that existing justification.
Now that you no longer identify as a witness, that justification is no longer there. Even worse for him, the reason for being hard on him you have rejected as wrong. In his mind he might think "Great, why NOW dad! Why not when we were kids - we could have had a normal childhood! All the restrictions and anguish! Why didnt you LOOK back then!"
To complicate matters, his wife is on his case. These conflicting emotions need to be vented somewhere and that somewhere is you. You copped it big time.
But isn't that what parents are for? Your response was really nice. Really nice. As parents we say that we would do anything for our kids, take a bullet for them if we had too. This is your bullet. Take it as a proud and loving parent of a son who is angry and confused. Taking it personally can only cloud the issue but looking at it as an opportunity to protect your kid might help you see passed the personal hurt and in the future he might see it as going some way to fixing the past.
Cheers,
Ob