I use fatty off cuts from the pet food store I get my meat from.. about 100-150grams on top of his normal food for a week will see him gain weight even if you are working him as per normal. Even an extra half a cup of kibble as well. When he is up a bit in weight gradually cut the extra food out until his weight levels out again... Gaining weight is easier than getting them to loose it, I hate cutting down the dogs food. Anyway good luck