He probably only has 20-25oz in a 24 hour period and he should be having at least 28oz.
Honestly if he's content between feeds and keeping to his growth curve then he's just fine and taking the right amount for him. The *should* in 24 hours is an average, you need to be guided by him. My 91st centile chunky DD never took more than 25oz a day (and that was a VERY good day) no matter what I did and she grew just fine. Even now at 7 she looks like she eats really well
but in reality she doesn't, she eats a lot less than her skinny 9th centile big brother
Don't start rice cereal, you will actually get less calories as the rice will fill his tummy with empty calories and mean he gets less milk. Formula is more calorific than rice.
I would try the faster flow teat again and see how he does but if he is able to go 4 hours between feeds just keep doing what you are doing.