Ex Fiancee wants to move miles away with our son. What do i do?

I am 33 and was with my fiancee for 8yrs, engaged for 2 of them, We also have a 2yo son, She is 26 and we were really happy together but just before she became pregnant things between us had gone through destruction, turmoil, heated fights, and plenty of hurt & heartache so we agreed that although we love each other we just don't seem to be able to be together & that its best for our son to have a healthy life without argument between his parents, We never saw eye to eye, I am now engaged to marry my new fiancee next February & my ex is now in a relationship with a touring musician and wants to move to LA to live with him and wants to bring our son. We mostly only talk when it comes to our son & i will always love her and want her to be as happy as i now am but it will kill me if my son is so far away from me. Any advice?