Drinking enough formula?

charity

My son was born three weeks early. Well since he was born (July 2nd) he only eats 2 oz bottles every 3 hours. (Sometimes he won’t even finish the full 2 oz) is he getting enough? I’m concerned he’s not but I’m wanting to get his weight up. He’s perfectly happy between feedings and has plenty of pee diapers but his doctor said he should be drinking 3 oz every four hours instead, but he won’t drink any more than 2. Thanks in advance