Eating it at once really isnt whats important its what they eat in 24hours. My first daughter never drank more then an ounce at once until she was over 6months old. But she would eat about every half hour lol. So it ended up being enough over all.
I think the general rule is half the baby's weight, so 4 ounces. But if that's too much or too little, feel free to adjust. And like the PP said, it's really more about what they eat over the course of a day, so just give him what he'll eat and feed him again when he's hungry.