Persistent diarrhea that lasts more than 24 hrs usually requires a vet visit. Is it diarrhea or soft stools? You need to make sure your little one is not dehydrated if it is diarrhea. Have you recently switched his diet? Any other changes? Does he act normal otherwise or does he seem sick?
Did your vet recommend immodium? I would never advise giving 'people drugs' to pets… I know that some are safe in very small doses, but I've never heard of immodium being administered?
After 3 days of diarrhea, I bet his bum is SORE! Poor little guy, hope he feels better soon.