You can register the POA without actually using it immediately.
I have POA for my uncle and aunt. I currently have nothing to do with their financial affairs or their health, but, should the need arise, I and the other attorney can invoke it, because it is regsitered. We would need to access the original and contact their banks etc., in order to use it, but everything is in place ready. Incidentally, they did this as a result of Mum's diagnosis. The are currently sound in mind and body!
It's not a dementia thing. What if your Dad, or Mum, had a stroke and were unable to write, or speak (even temporarily)? How would you (legally) manage their affairs? I would talk to your parents together, and take this angle. It's not about sidelining your Dad from making decisions, it's about insuring that, if the time comes, you are able to step in. He probably thinks that your Mum will just take over - especially if everything is in joint accounts - but it's not always that simple.