He's had multiple post-divorce marriages? He's probably over you, but just is very litigious and has a favorite victim in you.
I went to a seminar a year ago that said there are some personality disorders associated with extreme litigiousness. These people go to court often, and not alwyas for divorces, but they'll go on other issues, too. Either by getting themselves sued & not being willing to reasonably settle, or by just feelign like everything can & should be settled by a lawsuit.
This is not necessarily a problem in ALL the people with those disorders, just that it's one of the ways those disorders can show up. An obsession with litigation, with argument, with winning the argument or debate.
OK... the big three were Sociopathic, Narcissistic, and Borderine personality disorders.
It might be worth it to you to look up these personality disorders and see if you can understand where he's comign from... not that you'll agree, just that you'll maybe be able to put 2 & 2 togehrer, come to realize that you really are NOT the problem (as he's probably been trying to convince you for years) and maybe figure out how to remove yourself from his target sights so that you are no longer his victim...
Unfortunately, there's not always a way to prevent this, but at least it feels better to understnad it. My husband's ex is one of these people, a Narcissist, and ... she just can't stop the argument... and we can't figure out how to get out of her sights without abandoning the kids, which would not be right fo rthem... so we simply count the days till the end of the tunnel (the last one graduating high school and ending the support gravy train that gives her teh incentive to continue the litigation) is in sight.