There is nothing a vet could do. Mum knows best and she will sense if there is somthing wrong with him and would cull him if she felt he was ill. A baby food with no onions or leeks is fine. You can give him porridge, stewed veggies ground up small, (no onions) small bits of plain chicken and tofu. In fact all the pups should have these foods to help them grow
If you were to remove him and take him to a vet mum would reject him as he smells odd and would most likely cull him anyway. His best chance is to stay with mum now. We have has ones as small as him that were soon the same size as the others so see how he does.