Are you just feeding him kibble? I always have kibble down but at night I open a can of wet food (same as the dry food) and split it with six dogs...they gobble it down. You also might try wetting a little of his dry food to see if that makes a difference. Did the vet really look into his mouth..could there be problems with this teeth?
If all else fails send him to me..I know how to fatten up dogs...
