
Have his teeth been checked by the vet? It may be he has an underlying problem and is unable to eat. If all is OK do you soak the food or is it fed dry?
There are some good moist complete foods such as Naturediet, Natures Harvest and Natures Menu (the tins of NM are better than the pouches IMO). Any of these would be a good alternative to a dry complete, but get his mouth checked first.