How is his activity compared to his siblings? Get a coffee bean grinder and grind up the kibble and either add a milk supplement or not and just have water close to it.
Give him Nutri cal a couple of times a day.
Mom may sense he is ill or he may be weak and she notices this so she pushes him away so the other pups aren't compromised, it is instinct and part of survival of the fittest.
This may be the pup has not had the ability to nurse as well as the other pups so he has remained small. Only time will tell, and your intervention. |