Sounds like a clear case of over feeding to me. At 8 months he has most of his growth and may need less food. Three cups of a concentrated chow sounds closer to what he should need to me. Try cutting back to a compromise of 4 cups a day, and knock off the gravy and canned chow. It is quite normal for a young Shepherd to refuse to eat as much as the owner thinks it should. No problem even if the ribs show a little.
For more, see the sticky at
https://www.askmehelpdesk.com/dogs/i...tml#post254171