How often is your baby waking up?

My 15 week old is still waking up every 2 hours to eat. He has never given me longer stretches. He is 16lbs. I thought by this age he'd be at least giving me 4hour stretches. He starts fussing in the crib every 2hrs and if I ignore it, it gradually builds up to a full cry, so I don't ignore it. I pick him up and when I nurse him he eats well and hungrily. Every 2 hrs. When I pump during the day I get a good volume of milk on each side - (around 4 to 5 oz) each side and he is chunky, so I know he is getting good quantity. How can I stretch these feedings out and get him to stay asleep longer? Would rather not introduce formula though. Help!