Feed him separately, all pets should be fed separately and observed during meals anyway.
Maybe feed him 3 or 4 small meals per day instead of 2. Do not free feed, in example: don’t leave food down all day. Add water. Long walks as tolerated.
Follow him around when he is due for a bowel movement, immediately scoop and discard.
He is a senior, I went through this with one of my dogs, they gradually get a form of dementia, they don’t remember when they last ate and are constantly hungry.
Talk to your vet about medication to keep him comfortable. It’s a cognitive disorder, not related to the food.
If your vet approves for him to come off the prescription diet. I recommend Zignature.
Ps: One of the side effects of anticonvulsants is increased appetite.
