I would suggest more frequent feedings closer to every two hours. More than six ounces will upset his stomach where as four ounces every two hours will get him the amount he wants but in a way it wont negatively affect him. But don't put rice cereal in his bottle. Grains and starches like that expand in the stomach. There is reason behind waiting until 4-6 months to start food.

Give less milk more often. Do not mix cereal in the bottle. It causes problems in some babies. Dont assume yours is safe because someone else did it.

Maybe mix the formula with a small amount of rice cereal to thicken it up a bit.