Expierment a bit. I feed raw mince to a large majority of mine. They thrive on it. but like you, i had a lurcher (and now his daughter) that would not put on condition. He did not favour the mince, he also had limitless access to waste resturant feed but kept falling away to nothing. i tried dog nut and he would eat them sweet and put on the pounds. I then put him on a high protein dog feed, like tracker 27% that was better again. so play around with the feeding a bit. some dogs are pickers and they can break your heart. Does he feed with other dogs? if he does feed him alone
best of luck