I live in MN. When my son turns 18 can he choose to live with me without his dad interfering? Do we have to go through another long drawn out custody battle?

Unless there is something in the order saying 21 he should be able to decide.
Is he going to go to college?

When he is 18 he is a legal adult, he can live alone if he so chooses.

If there is a custody order that extends past age 18 you will need to file for a modification. But at 18 he can live where he wants.