In Minnesota,what age does the child need to be to be able to say which parent he wants to live with, if both parents are fighting for him. One wants only legal custody and one wants both legal and physical custody.

There is no set age, but perhaps as young as 12 the child's input may be listened to.  At any age,the child's needs will be considered.  If only one parent wants legal custody, it doesn't sound like much of a contest.  If they are not both seeking physical custody, there isn't much to talk about.