Is he pining/crying? Is he allowing cuddles, petting? Does he seem in pain when he's patted or cuddled? Can you take him to visit his other dog friends at all? Or can they bring one of them to visit with you?

Is he drinking water? Is he going to the toilet? If he's not going to the toilet, or seems in pain when touched, then call a vet.

Sorry for the 20 qu - just wondering if he's homesick or sick. If he's absent from Mum and his pack mates, he may be pining quite badly. Unfortunately dogs don't understand about being left or what "later" means. Also, it depends on his history - if he came from a pound, he may be insecure about being left again.

GL