In the meantime, yes, he needs to beef up a little. I find that dogs love cooked chicken
and cooked
ground beef added to their foods, along with cooked rice. (I won't get into raw foods, they have merit, but it must be done with care and great understanding of fresh, raw feeding).
See if your dog like the addition of cooked, shredded chicken or ground beef. Sometimes simply adding cooled chicken broth to his regular food will help.