You probably aren't going to be able to explain it so he will understand. This all goes hand in hand with dementia/alzheimers. I would listen to what he says (not that most of it will make sense) and then make the decisions that are in the best interest. I wouldn't keep bringing up that you have power of attorney.
Good luck!
Dont say it, it is hurtful.. anyway steer clear, talk about positive things, let him still have some contorl if he can, that wont hurt you will it. Good luck