My lo only took 2-3 ounces at that age, but she was 8lbs. Babies love to suck, so when he is done with the bottle (which doesn't require as much effort to get his milk) he probably still wanted that comfort. Breastfeeding requires the baby to work harder and they get their need to suck from the breast. When you give them the bottle babies will eat whatever is in the bottle because they are used to sucking much longer on the breast (just as you said). I think 6 oz is ok since he is a bigger baby as long as he is not spitting up.