You should be able to feel individual ribs, but not see the definition of each rib. This is a good rule of thumb. If you don't feel his ribs, it means you're overfeeding him. If you see visible ribs, then your dog is truly hungry and needs more food.
Do dogs know when they are full?
Dogs know when they're full or not, they'll eat more when they're not satisfied with their food and vice versa. The dog's digestive function is a bit the same as other animals, and they get stuffed when they overeat. However, dogs will eat anything presented in front of them due to the “feast or famine” mentality.
What happens if a dog doesn't eat enough?
Although a loss of appetite in dogs doesn't necessarily indicate serious disease, prompt veterinary attention is important because it could be a sign of significant illness, including cancer, various systemic infections, pain, liver problems, and kidney failure.
