He might not be eating because of the bad teeth. But he also might not be eating due to depression. As an older cat, now abandoned by his family he could feel quite insecure.

Will the shelter be getting him to the vet for dental work? I would think that would be an important first step to making him feel better.

I do not know about a food called NuPet, but try canned foods that are ground texture (not slices or chunks) A kitten canned would be the best because of extra nutrition/calories.

A vet could possibly get you some canned a/d which is for this type of problem.

Also, get some nutri-cal paste. At least it will get nutrition and calories into him until his appetite perks up.

Try also, some baby food meats. Not overly nutritious, but they like the taste. If he DOES eat it, you can them add canned cat food to it slowly adding more cat food and less baby food.