
The vets I work for tend to ignore numerical weights, but go by appearance. When you stroke his sides can you feel his ribs without too much difficulty? If there's no fat over them, a dog's too thin. If you need to push hard to feel the ribs below a layer of fat, the dog's overweight and should be fed less; in this situation cutting the total food quantity by a quarter to a third would be a good start. Castration is notorious for encouraging weight gain, and castrated dogs generally need
much less food (about a half) than an entire dog.