| People with performance dogs that have high calorie requirements usually supply those extra calories through fat. Fat has a lot of calories but very little in the way of nutrition, so it can be fed without upsetting the balance of the kibble.
If your dog tolerates vegetable oil, you can try adding some to his food. Some dogs are sensitive to too much fat, others will take as much as you can feed them and be fine. One tablespoon of most types of vegetable oil has approximately 120 calories. If it doesn't upset his digestion, working up to 3 or 4 tablespoons per day would give him the extra calories to put on some weight while not upsetting the balance of the Canidae.
Another option would be to switch him to a performance dog food that would have a higher fat and calorie content |