How to get approval for relocation?

I have custody of my two children. There are no geographical restrictions in my divorce papers. He was ok with me moving but his parents are making him fight for me to stay. I am engaged and my fiance got a transfer to another city in Texas. He states he will take me to court and take my children away. What do I do? There are great schools, great athletic opportunities for my son who plays baseball. He only sees the kids when I have to work and his career is far more important than the kids. This move will be better for us because I will have more one on one time with my boys and I will not have to work the weekends. Please help.