I am not a senior member, however I can add my experience and imput.
OP, you've already been given IMO, the best advice your gonna get.
YOU can think you ex is a sociopath, and maybe he even is one. But it does not matter. The only time it may matter is if the COURTS order a full on custody/psychological evaluation of BOTH of you, and deem that your ex is indeed a full blown sociopath. Even then, its totally up to the psychologist to evaluate what he/she thinks is best for the child in terms of custody, visitation, etc. If you don't like the psychologists recommendations, you can spend zukoo's of thousands of dollars on expert psychologists and get a trial going...
basically, unless there is a diagnosis that your ex is a sociopath, you need to just let it go. Do what everyone is suggesting...file for a modification, don't stress about the grandparents - they have no rights, and if you have the bucks, request IN COURT, a pyschological evaluation for both of you.
I am writing as my ex does have issues, and you know what? In the eyes of the court, unless he is proven to actively harm our child, it does not matter.
Its a hard pill to swallow, but speaking from experience, the sooner you come to accept your ex the way he is, the easier your life will be. Follow through on the legal process...get the modification happening based on lack of visitation and based on your childs current medical needs. Request a custody (psych eval)evaluation at the same time. Then take it from there.