I've been thinking about your moving. I know his family is very 'involved' in this whole mess. You do need to consider the likelihood of them/him going to court if you relocate. Are they the type to put their money where their mouths are and file for residency or fight to have DD with him for long periods of time? BFF's ex wouldn't have had the money to engage in a long distance court battle to try to force her to move back to CA from TX.
I agree that, especially if you are thinking of relocating, you need to start building a body of evidence of his/his parent's abusive behaviour. Start a notebook/diary and write down everything they say and do that crosses the line into insulting, abusive, intimidating, or threatening behaviour and report threats and intimidation to the police, every single time. If you see the derogatory FB crap, screen shot it. If people tell you he said 'such and such' write it down or ask them if they'd be willing to give you a written statement. Include dates, times, places.
As far as child maintenance goes, if you are wanting to relocate it's probably better that he isn't paying. It's simply more proof that his child's welfare is NOT his priority. Obviously if he's not paying because he's not working it's a different story. But if he works and doesn't pay, use that fact to your advantage.
Finally, I know you've said that your family is not involved, but if you feel that he/his family might go down the 'legal route' if you move, it looks better if you relocate to an area where you have some type of ready-made support. Give it some thought to see if there is anywhere like that for you, even if it's a friend or distant relative in the area.