Just to clarify an earlier answer, in Minnesota a witness can receive property distributed under a Will without any consequences to the Will's validity. This post has been for educational purposes and should not be conisdered legal advice. Attorney is licensed to practice law in Minnesota.
Every state is going to have their own law about the requirements for an effective POA and what powers it conveys to the attorney-in-fact. I would suggest that it wouldn't hurt to have one executed in each state. If you think you are going to want to sell the home during his lifetime, then it would be a good idea to have one done in AZ, otherwise the state where his bank accounts are located would likely be the most important state to have one done.