Muy pit is 4 years old. When he was 2 he got parvo and survived. Now he won't gain weight. He weighs about 65 lbs. and should be at about 85

How skinny is he exactly?

Can you see a few ribs towards the back of his body or can you see his whole spine?

Seeing ribs is healthy, it means the dog is actually in good condition.
Most people assume that if you can see rib then the dog is skinny.

If however you can see more than the last few ribs or spine then I would try putting him on a higher protein dry food.
There are many available on the market for underweight dogs.

You can also try feeding lamb flap along with his regular food.
Lamb flap contains healthy fats that can help them to gain weight and it is cheap, however if you do decide to do this make sure you only feed a very small amount at first and build it up