Did you get a health guarantee with your pup? I know you are already in love with him-but if it is neurological, he will have to wear diapers all of his life. I would suggest getting a second opinion-though your vet is most likely correct. Contact the breeder-if they are honest people, they would take him back and replace with another pup.
They knew what they were doing, as they had him for 5 months.So sorry to hear this.