Blue heeler mix lethargic, shedding, no appetite

by Anna
(TX USA)

I have an adult middle age blue heeler mix that has within the last week become very lethargic, with little/no appetite, and has begun shedding profusely. He mopes around and will eat soft food out of my hand but will only nibble on his own. I gently rubbed his head/ears/chin and he whined as if in pain; I noted no swelling. His nose is cool and wet. I feed him Purina (wt. watcher) dog chow. Please advise. Thank you in advance.