I would make an appointment with a Veterinary Internal Medicine Specialist and get an accurate diagnosis.
There is obviously something wrong with this dog that a simple dietary change cannot fix.
Probably why the dog was given up.
I would not experiment with trying all types of different foods as you can make the problem worse.
You need a specialist to get to the root of the problem, asap.
PS: Don’t underestimate his aggression issues, keep him away from children and other pets.
Trainers are not licensed veterinary healthcare professionals.
And NO, I do not think giving puppy food to an adult dog is a good idea.
Will it cause him harm? Who knows…
Again, consult a vet that has examined the dog.
