zippey "Can you ask your family to move closer to you?" So now the OP's parents as well as the OP need to dance their lives around the dad.
"I know the OP doesn't paint her ex in the best light but we only have her words. His point of view would be much different."
That's the thing with Mumsnet, you only ever get one side of the story. Always.
"I just think it's almost evil to take a child away from a parent without so much as a consultation, mum or dad." She is not taking their son away from his dad, she is moving closer to her parents, commutable distance. No real issue. Unless the ex wants to make it so. Now why would he want to do that?
So he could see more of his child more (who he is not seeing much of now - even though he is very close geographically), or simply to piss off and control his ex! My guess is on the later!
"...how to come to a compromise." What compromise should there be on a man controlling where his ex partner can live!
Can she control where he lives? I can see if she was moving a long way away that this would be an issue but the OP can make it very easy for her ex to see his son.
The OP can have those ideas of how contact will work and present them when she needs to. I would suggest OP you work out how the relationship will be maintained and work at its current level and tell him when the time is right. Do get legal advice to avoid any nightmare scenarios, but do no be scared by ideas that he is a super dad and will be able to prove this, it is only a problem if he genuinely is a great dad but from what you say he is not.
Trollspoopglitter "He's already suggested he has his son full time, and will most likely be able to prove evidence of a stable, supportive environment in front of a judge."
Where are you reading that he wants his son full time? Where are you reading that this man who makes paying maintenance and seeing his son difficult for all be, able convince a judge that he is a good dad?
"Let's say you move without telling him. He goes to court. Try to imagine how he can make it look. Do you have any evidence he or his family is controlling/bullying you? Is it just your word?"
Good point, get all your evidence, screen shots of emails and texts, encourage all contact to be through email, text or recorded message, if you can, so you will have evidence of what he says.
"He can tell the judge you're the controlling one and won't allow him to see his child." But only if that is true, if you are making access to his son easy for him and he is making it difficult - get a record of all this, and also of how he is not making getting maintenance difficult.
"His proof? He wanted a partial custody, as he felt you weren't mentally coping. You refused and then moved his child 100 miles away without informing him - at all. Can you see how that looks to a stranger who has two completely different versions of events being told to him?"
That is a good point but only if the dad is able to prove he asked for partial custody, I am not sure he did that at all. His parents, I think, offered to take the child all the time, which I would see more as a threat than a seriously kind offer.
Do you have evidence of his parents wanting to take your child? Any emails or messages, OP?