Your ex can apply for a prohibited steps order to prevent you moving - but it's unlikely he'd get it if he only sees DS infrequently and if you have a sensible proposal as to when he can have access during holidays and have contact with his dad outside of that (email, set time for phone calls or skype) and offer to meet him in the middle of the journey to cut down on travelling times.
I have residency of my 8 year old son. My new husband has just been promoted up north but I know my ex isn't going to like it. Although he only sees my son when he feels like it. Can I do this or can he stop me.