I just brought my 8 month old son to the pediatrician and she told me no more than 30 ounces per day (averaging 24-30/day). Not including formula in the cereal. The only drink should be formula - no water. Always give formula first.
I give a bottle after waking at 6a (4-6 oz), after morning nap at 10-11a (4-6 oz), then after the afternoon nap at 2-3p (6oz), then 6 oz at 4:30-5p, then 4oz at bedtime. I feed him solids at least 1-2 hours after the formula at breakfast, lunch, and dinner. It is usually within the hour of naptime/bedtime. He seems to be fine with this feeding schedule.
But as most will say, every baby is different. If your pediatrician is aware of the decreased formula intake, and he/she is not concerned, then there's no reason for concern. They look at your baby's weight/height at each visit to determine if there is a "failure to thrive". Make sure he has at least 6-8 wet diapers per day to know that he is not dehydrated.
He does get some fluids from his food. Calorie-wise: since formula is 20 cal/oz, you should try to feed him calorie-denser food if he is not drinking enough formula. Examples: plain whole-milk yogurt, bananas, earth's best has lentils (it's mixed with carrots, apples and rice) - which is 90 cal/4 oz. Make sure he has had the 3-4 trial of all first. Look at the jarred food to see which foods are calorie-rich.
Good luck to you!