I agree completely with TeresaM. Cash used to be very fussy eater until I started looking after my sister's labrador (who eats very quickly and like a garbage disposal

). Now Cash will eat his food (I don't free feed) when I feed them and although he is slower, he finishes everything eventually.
But I have another tip for you. Try to get a supplement (I use Nutureline's Show Dog) to put on the food. It helps them gain a little weight and makes sure that they get all the necessary nutrients and it is wonderful for their coats. So, even if you feed your baby chicken (mix it with some dry food for his teeth), you can mix the supplement into the food and he will get all the needed nutrients.
I hope this helps.