Has your son been baptized yet? If not, now is the time to try and reach him. Does he plan on getting a college degree? The Witnesses discourage this extremely. Try hard to think of the things that might be red flags for your son and strss them to an extreme. Once he is baptized, the game is pretty much over.
I take it you have shown him sites such as this and others. Are there children involved yet? Remind him they will not have a normal childhood, like he did. Does he want to put them through the hell of being a total outcast at school? No birthdays, holidays, sports. The daily embarrassment of not pledging allegence to the flag. And all this begins at an age when they really don't understand why, but feel intensely the emotional pain of being different.
I was not raised as a Witness, but I have a son in his 30's who still has nightmares about Armegeddon. The damage to children can be severe. I raised him as a Witness, and am sorry now. I wish someone would have played on my doubts way back when.
Always talk to him on the basis of concern for him and his children. Don't be harsh because it plays into his persecution complex the Witnesses thrive on.
Best of luck to you. Sincerely, Choosing Life