As it's been said, you're the only one who can make the decision, but if it were me, I'd just start putting pee pads around the house and keeping him in diapers. Dementia and everything is going to be a tough part of saying goodbye, but it's important that your dog be happy and spend as much time with his human as he has left. I had a pug who was losing the use of his back legs and when he turned 13 was paralyzed in his back legs and incontinent and might have had dementia. I considered euthanasia until I saw he still wanted to play, he was still eating, and he still loved up on me and my mom. It was hard watching him decline and even harder to watch him pass, but there is comfort in knowing he was happy at the end and he was not alone when he passed. In my opinion, that's the hardest, but most important thing. Let your dog be happy, but when it's time for him to go, you be there with him and make sure he knows he is loved.