Have you tried any Dehydrated food? Ex: The Honest Kitchen or I and Love and You etc. Some are expensive but if
you buy the 2# box of THK to try him on it, it lasts quite a while. depending on his weight. Our 2# box lasts a month with 2x daily feeding and my dog is a havanese, but 20#. We rescued him at 16 lbs. 8 years ago. He was fed the dehydrated foods since we rescued him. Tried the all one time or another. He ate them all, liked the moistness I think. Anyway, you can
add meat, veggies etc. ON OCCASION for a change. Hope this suggestion helps.
