Is he eating some of the food just not all of it? Are you leaving it down for him to graze on? What is his weight like and is he generally well? Is it just last few days when its warm?
Assuming he is well, weight is good and he is eating some of most of his meals I would just be feeding meals, using his kibble for training and not leaving food down for him during the day.
If he is really struggling you could try switching to a wet food instead but that is a whole new minefield.