Probably best to work out the total calories in the amount of puppy food he's currently on off the info on the bag and then work out how many calories are from protein, carbohydrates and fat respectively by their percentage content. Roughly, 1 gramme of protein = 4 calories, 1 gramme of carbohydrate = 4 calories and 1 gramme of fat = 9 calories. Chicken meat is probably about 20% protein by weight when cooked, not sure about rice, but that will be on the packet.

Not sure if I've explained that very well, but you'll be able to do it fairly accurately.