I really wouldn't worry about how much they are *supposed* to get, just offer bottles slightly bigger than you think he will take and he will take what he needs. If he is showing hungry cues follow his lead and feed him. If he is alert and active when he is awake, sleeps well, has plenty of wet and dirty nappies and growing he is getting what he needs!
My DD was 8lb 11oz born and gained weight really well, but she very, very rarely took more than 25oz a day even when her weight said she should be taking 36oz+ a day and she stuck like glue to the 91st centile so was clearly getting what she needed.
He will probably increase his feeds soon, particularly when he gets used to the 4 hour EASY. What flow teat do you have him on - try a faster one if he's not on the fastest yet.
Laura