Ragdoll Kitten with senitive stomach 15 weeks old?

My Ragdoll Kitten 'Dexter', I've noticed Dexter has had diarrhea and bloody stools lately, I have reported it to the vet, I have fasted him for a full 24 hours and feed him chicken and rice, which he did become good, and then I've tried mixing in Purina one kitten food with the chicken and rice, to get him back on the dry food. I notice diarrhea again, did i change the food too quickly? How many days do I have to slowly change the food? If some one could make a suggestion that would be great. I'm really concerned, this it my first pedigree kitten.