I'm taking him to the vet this afternoon to be put down.

He spent the entire weekend by the waterbowl. Didn't drink with his tongue, but kind of dunked his whole chin in and he was all wobbly the whole time. I'm guessing because he's so weak. I gave him a small portion of wet food, which he dove into, but it was so sad to watch him eat. It was getting caught in his mouth, gagging him. We managed to pry his mouth open later to see if we could see what's wrong. All of his front teeth are gone, which is why he can't eat.
I'm heartbroken. He's been ours for 16 years.