Toto wouldn't eat the first few days we had her either ... she was 16 weeks. Our breeder sent home enough of the food she had been loving to last us probably 5-6 months, had she eaten it!
It was the frozen Bil Jac and it smells awful ... I called her back to make sure it wasn't spoiled!
She weighed 1 lb. 8 oz. the day we brought her home and lost 3 oz. the first day ... I took her back to the vet 3 times that week. She had us give her a little peasized drop of Vet Formula Nutri-Cal 3-4 times a day for about 6-8 months. She never ate a bite of the Bil Jac for us and is, to this day, the pickiest eater you ever saw but has always been disgustingly healthy!!
I would say if he is at least eating a little something but drinking plenty of water, he would be fine. They dehydrate very quickly. Good luck and please keep us updated.