I had a dog who lost about 3-4# every summer because she didn't eat as much. I never worried about her. Have you thought about feeding him only once a day? He may be leaner because he isn't eating as much (unless you are making up for the missed breakfast with more in the evening).
If his poop/pee is fine and he's as active as usual, I think he's ok. Now if only
I would eat less in the summer...
