Have you got any legal advice at all?
I wanted to move away for similar reasons. Mostly be near to family support as I had no one where I lived with my ex.
The legal advice I received was that the furthest away I could move was about 1 hour. Unless of course he agreed to me going further away, especially if they have established visitation and time spent together.
It sucks, but you could try. You would have to go to mediation first and then to court etc etc