K.A.
Hi M...I didn't read through all the responses, so I don't know if anyone has mentioned this. But, I would be careful in letting your ex insure your son’s car.
My dad told my brother if he paid for his own car, he would take care of the insurance. He ended up letting the policy lapse and my brother did not find out until he was in an accident and attempted to file a claim. I don't know if your ex would be that malicious (my dad did it intentionally to be a jerk and teach my brother "a lesson in life"), but since your ex hasn't kept his word in the past it may not be worth the risk.
I tried to talk my brother out of trusting my dad, but just like your son he thought I was trying to hurt him not help him. He still has debt from the accident and lost his license for a few years due to being in an accident with out insurance. I would try to explain to your son the risk of trusting your ex to continue paying the policy by pointing out the times that he has not followed through. Also, let him know of the risks involved if the policy lapses and he unknowingly continues to drive.
Also, my brother is currently using my 2nd car (title is in my name) but has a separate policy that is only in his name so I know that it's possible. Good luck!