You will have to have the agreement of the child's father in order to be able to move him out of the country. After much negotiation, my ex provided me w/a notarized letter stating he had no objections to my moving to the UK w/our kids. That was sufficient to get visas for my girls.
If your ex says no, your options are pretty limited. You can go to court for sole custody, or you can try to get a judge to order that your child be permitted to move to the UK. Impossible to say how successful that would be, as it's dependent on your personal circumstances.
My advice (from personal experience), be upfront w/your ex and be patient w/him. Try and give him as much time as you can to get used to the idea. No matter how unreasonable you might think he's being, moving a child out of the country is a really big deal. Make nice as much as you can, and don't bring out the big guns unless/until you need to.
Good luck!
