Anyone experience odd behavior with senior cat?

First of all, I am very sorry to hear about your cat. He is without a doubt uncomfortable, I doubt he suffers from dementia. He is more likely very uncomfortable and is looking to you to relieve his distress. Does he seem hungry? I would think his appetite is pretty suppressed and what he really wants from you is just comfort. Make sure he has plenty of water available to him, and food, but more importantly, spend some quiet time with him and reure him by petting him and talking softly. If he needs it, carry him. Good luck. My heart goes out to you and to him.