First of all, who has custody of your son? If you have at least joint custody, then he shouldn't be forced into jw activities/beliefs while with you. Having proof of your ex's relationship is a HUGE bargaining chip for you to gain custody if you don't have it.
Second, your ex is bound by secular law, so he isn't head of you and wasn't even when you were married unless you let him. He sounds like a hypocritical, sanctimonious asshole.
Get custody of your son if you don't already have it, and limit his association with the Watch Tower cult as much as possible. Risk of being reported to the elders should be great enough for your ex to comply. And do not give him freedom to remarry based on a lie. I wouldn't even give it to him if/when you're intimate with someone else. That is nobody's business but yours.