It very likely is due to the transition to richer food. As long as he is otherwise normal and drinking enough, he is probably just fine. Ensure he is not eating too much at a time, and perhaps give him some plain yoghurt [a tsp. or two at a time] to help with digestion as his system gets used to the new food. There are a few dogs that take months to adjust--but it is worth it! Most people give up too soon thinking the dog cannot handle the new food. Sometimes mixing the two foods causes more trouble than switching more rapidly. Unfortunately each dog is an individual, and we have to try to figure out their inner workings.
Check out the other stomach related thread going on right now: