Mary it is possible the diet change could be the cause for the diarrhea. Other possible causes are: Intestinal parasites, ingestion of a foreign body, infections, or even part of a systemic disease to name a few causes. This is what you can do:
1- Do not give things like Pepto Bismol or imodium. You see, stopping the diarrhea with medication can make things worst if this is related to an infection or ingestion of a foreign body.
2- No food or water for 8-12 hours, need to give gastrointestinal tract time to rest. Then, offer small amounts of water followed by a diet of rice (75%) and boiled boneless chicken
or ground beef (25%) until the stool is back to normal then wean back into regular diet.
3- Supplement with a probiotic (fortiflora), these are friendly bacterias that help with regularity. You can buy this over the counter in any pet store.
4- Consider getting him dewormed. You can buy Panacur (fenbendazole), give it once a day for 3 days, repeat after 3 weeks. You can buy panacur in some pet stores, online (amazon.com) or vet's office.
5- If after doing the above you do not see improvement within 24-48 hours, have him checked by a veterinarian. My goal is to provide you with excellent service – if you feel you have gotten anything less, please reply back. I am happy to address follow-up questions. Thank you for your business and I hope to work with you again soon!