I donn’t know what to do anymore

I have had my dog for 15 years, he is a poodle mix. Always been a pretty good dog until this last year or so. He doesn't do anything but mooch for a treat. I am not kidding, he does not want to play, he does not want to go outside, he does not want to hang out. Nothing. It has gotten to the point where he is now pissing and shitting in my house, trying to skip his dinner in favor of a treat. He has never been given treats just because, always after a poop or something that he did good. So I don't understand why he is doing this now, he doesn't listen to any of my commands anymore. He bothers me for a treat from the second I get home until he finally falls asleep and honestly I am f***ing over it. I figured he was bored or something but he literally has no interest in going for walks anymore, or even going out in the yard. I took him to the vet in fear something was wrong and he is fine. Perfectly healthy for his little old man age. Help me help my buddy =(