What would you do??

Divorced since Jan 2015, was then taken to court for shared residence which was denied and I was given full residency with the ex allowed access as and when the children (15 & 12) wanted. We reconciled in Nov 2015 and spilt finally in April 2017 when I discovered he was seeing the person he had a relationship with when we originally split.
Hasn't paid maintenance since Jan 2017 so contacted CSA in July. Still not paid and now they are doing direct pay - due to start in Jan 2018.Ex has now contact CSA to dispute parentage shich has absolutely gutted me as we were together 22 years. History of Domestic (emotional) abuse from ex and had non molestation and occupancy orders against him and now it has all indirectly started again with facebook rants and messing with my sons head. Daughter (15) doesn't want to see him but son adores him.... would you stop him from seeing kids as I think he is unstable or just keep plodding through all the bullying and abuse. He is living with the woman he was seeing and I have a relatively new relationship with someone who makes me very happy.