My 8 Month Old Eats, Maybe, 36oz Of Formula A

My 8 month old eats, maybe, 36oz of formula a day and had been eating rice cereal the first couple of months. Now he's eating oatmeal cereal and we have incorporated fresh fruits and veggies but since last week he has had diarrhea which has smelled like sewage. He hasn't had a fever but his bowel movements have become more loose than when we first started fruits and veggies... Is it necessary to take him to the doctor to be checked out?