Use THEIR tactics.
Be proactive in telling your son how cult leadership tries to destroy families by demonizing one of the parents. Tell him how they twist the bible to get him to be suspicious of you. THAT's how they begin to control his mind. Every conversation you can conceive that your ex and the elders might have with your son, YOU HAVE IT WITH HIM FIRST to prepare his mind for the onslaught.
Don't let yourself be caught in a reactionary position. It will play right into the hands of the Watchtower recruitment machine. Watchtower is very adept to picking apart a person's objections and reactions - tying them to Satanic influence. What they are not used to is dealing with someone who is IN THE KNOW from the start.