Thank u for rescuing this baby
First off, have you considered a holistic vet in your area?
You can home cook for your dog, it's easy and may solve a lot of your problems. Processed foods are just that, processed.
If you look at your dog food label you will see by products/lamb meal more then likely. The process for rendering down meat not fit for human consumption but for dogs is disgusting.
Some food is obviously better then others. But you may wanna try the home cooking.
Supplement wise I'd say some vitamin e, fish oil and shark cartilidge for sure.
Do you have any pics?