Run full internal parasite/tick/thyroid panels on him and if he’s clean, start throwing -good- food into him.
I recommend EVO grain free [beef] and if that’s too steep, try Taste Of The Wild.
They have several varieties and my dogs prefer High Prairie.
It’s available at all Tractor Supply stores.
he’s bug free, and looks a little better. We just got him on a good food, and my dogs loved Taste of The Wild, too. We’ll have to look for it where we are now.