Can be a number of things his diet, worms or a general jippy tum. As vet suggested leave him off for for 24 hrs and then try him on small bland diet chicken and rice or white fish is also good and scrammbled egg as its binding then gradually get him back to his normal diet. If this returns your know his food is not suitable for him. If hes still bad in a day or two take him in to be checked over as they can dehydrate .