He may have the canine flu and he probably picked it up at the shelter. It takes longer to get over than kennel cough which is a milder virus.
www.marvistavet.com/html/body_canine_influenza.html
You can see if Tamflu will help him shake the viral infection that led to the bacterial pneumonia. Pneumonia takes a good three weeks to get over.
www.marvistavet.com/html/body_oseltamivir.html
You may find bringing him in a steamy bathroom will help his breathing or running a vaporizer with just plain water in it may help.
You can see if some NutriCal will help perk up appetite as its very nutritious and full of vitamins and minerals. You can put it right in his mouth.
Based on what I've seen of shelters it is likely they will just euthanize him if you return him as they seldom have the resources to pay for medical care for dogs and they would not want him to make other dogs sick too.
You can boil some chicken meat and see if he will drink the broth and maybe eat some chicken if he is off his food. You can warm up his food so its more aromatic as if he can't smell the food he won't eat it.
I would suggest seeing the vet again or at least asking about using Tamiflu to work on the virus part of his infection.
Hope this helps you!
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