Is he draining his bottles? If he is you need to put more in them. With a bottle feed there should be a bit left in the bottle at the end of a feed so that you know they have stopped becuase they were full and not just because the bottle was empty. With cluster feeds, offer a full feed and let him take what he needs.
By a month a 4oz feed in quite a normal size feed, some babies wil take a touch less some a bit more.
HTH!
Laura