It is so hard not to engage! Been there done that!
So... what worked for me was no contact... zero contact. That cannot work as easily with children but the way to handle that is to have very specific boundaries on visitation. Very strict agreement in writing on how, where, when etc... and that will minimize discussion. Any other matters relating to son and visitation are to be done in writing only... no discussion of any other subject... period. Put it ink and concrete. Ignore anything he tries to get to bite with... hang up if he calls. Don't engage. ever.
Best case scenario is peace and serenity and opportunity to detox from him. He may actually seek help because "you are the problem" but as long as he see's through his "pair of glasses" he won't get anywhere. Don't get sucked into the manipulations of a person not in recovery!
Sounds like you are doing GREAT! Take time to have fun and find something that makes you laugh... its good medicine for the soul.