The recs for bottle formula fed babies is a minimum of 20oz up to 1 year, after than they need 12-14oz of dairy to get their needs met. Most babies will still be on 3 bottles up to 1 and then drop to 2 bottles at 1. I would offer a full 9oz bottle for now and see how he does, if he is taking less than that at every feed then cut the bottle back an oz or so.
HTH!
Laura