What cat food is good for indoor senior cat with a sensitive stomach?

My cat, who is 18 years old, has recently developed quite the sensitive stomach. He has an upset tummy nearly every day, I think its probably his food. Im trying to find one that fits all 3 qualifications, but I cant seem to find a cat food for senior and sensitive stomach. Hes been eating Iams indoor senior formula, and Im considering switching him to Nutro Senior, because it says its easy to digest. Has anyone else used Nutro, or have any suggestions for a good brand of senior sensitive stomach food?