he's 8 months old - is he eating solids? pablums, fruits, starting on vegetables??
If he is drinking up to 30 oz. of formula plus eating breakfast of pablum and toast, lunch of fruit, and dinner of vegetable and a bit more fruit, plus snacking on juice and soft foods (bananas, toast, plain yogurt, etc...) then don't worry - he is getting enough.
BUT... if he is not eating, and only drinking up to 30 oz of formula per day - then I would suggest taking him back to see the doctor and discussing your concerns and fears.
He was born weighing 7 lbs 11 oz. which means that he should be near 22 lbs at one year. So it sounds as if he is not that far from his weight goal.
I hope that this helped some. I am a mother of three. They are 10, 3 1/2, and 1 1/2. And I understand what it is to worry that they are not eating right. Take care and keep us informed of how things are going with you and your baby.
Peace