Agreed about the weight. If he's lean but not losing weight with a regular diet then you shouldn't worry. If he is losing on a regular diet then worry. 10 pounds is 25% of a 40lb dog's weight. That's the equivalent to about 38 pounds on the average 150 pound human. So that's like a lean person suddenly wanting to add 38 pounds. Not something that should be done real fast by simply overeating.