http://www.kellymom.com/bf/pumping/milkcalc.htmlHow many times is he feeding during a 24 hr period? According to kellymom.com, exclusively breast fed babies require anywhere from 19-30 oz per day, with about 25 oz a day being the average. Just divide that into the number of times you're feeding him, and you should have a good idea of what quantity you should be giving at each feeding.
He could be wanting to nurse more often for the comfort factor too. Maybe he just likes feeling close to you.
When he does nurse, you could try doing breast compressions to encourage him to keep drinking, and that might help him last a bit longer in between feedings.