I have an 11 year old son and his father is saying at 12 he can choose who he wants to live with.

The father is wrong. A child (under 18) cannot not make that decision in ANY US State (and most other places). Only a judge can modify a custody order.

Some states do provide guidelines of how much weight they place on the child's preference, but a judge is not likely to place a lot of weight on a 12 year old's opinions.

In any case, the father will have to petition the court for a change of custody. The court will then hold a hearing, part of which would be talking to the child and then hand down a decision.