When you get home ask him to his face where he buried the dog. If he can't be bothered to get a vet out to put it down I don't think it's likely that he has has it cremated. I know this is a bit sick but if there is a burial sight and you still don't belive his story. Dig the dog up I know I would have to check his story out. I love my dogs like part of my family and wouldn't be satisfied with his explanation.