Lavender,
There is a slow decline in people entering and a big exit through the back door. That may not matter to your son, though. The Witnesses will just use it as a sign that the end is near. They manage to do this no matter what happens.
Best thing you can do is continue to show him unconditional love and place a well thought out comment from time to time to try and reach his heart. Is he pretty much won over by their beliefs now? If so, they have taught him that Satan will try to stop him by using his family to weaken his faith. They get you coming and going. That's what makes them a cult.
Stay as close as you can to your son and he will know you are always there for him. I don't remember if you have been pointed toward the book, Combatting Cult Mind Control by Steve Hassan. He is an exit counselor and went through the cult experience personally so he can see it from both sides.
Unfortunately, there may be nothing you can do at present. Don't give up hope though. Many of us woke up after 30 years or more. Each person has to be ready to see the things that they are blinding themselves to at present. It may take a tremendous amount of patience on your part. He is lucky to have such a loving mother.