I would continue with the chicken and rice mixture and you can even go so far as to switch him over to jarred baby food. Just choose a product with no onions or garlic. Jarred baby meats work great because they are bland. This will help with the diarrhea.
If it persists, you can give him some plain pedialyte to keep him well hydrated.
You might want to see the vet for a fecal to rule out parasites, the number one cause of diarrhea in pets.
This fecal runniness could be because of a change in his diet over the last few weeks.
Let me know if you still have questions.