If I were to start a conversation with him, I would initially focus on two things: I would recognize his value and dedication and I would get to know him better. I don't know his name, but I'll call him Ben. I would probably start like this:
"Ben, I know this whole thing is very hard on you, but let me say I think you're doing a great job. No matter what anyone says, you're here for your father. That means a lot and I am sure your father would be proud of you for that. Not only that, but when the hospital asked you to step up and take on an important responsibility (power of attorney), you did it without hesitation. You're a loyal son and a gentleman.
"How are you getting along? Have you ever experienced something like this before? [Wait for the answer and talk about it. Let him talk about his experiences for a while and choose to go in more depth about a particular experience.] What was it like to go through that? Did everyone turn out OK?
"Listen, Ben, I'm here for you. I want you to keep the power of attorney because you've demonstrated your loyalty. Can we work together to figure out what it means to have power of attorney? I'm not too experienced with it, but I know attorneys in general have certain responsibilities to communicate with their clients and I'm sure something like that applies here. You've taken on a responsibility to communicate what's going on and I want to help you fulfill that responsibility.
"I know your family has been hard on you, Ben, so I want to help you by communicating with them for you so you can focus on the more important things. Again, I'm with you every step of the way, so let's figure this out together. We're going to get through this."
As you work with him, always be on his team. Never his superior nor subordinate, but always his peer. Navigate together.