Please help! How do I pump without starving my baby?

I feel like this is probably a dumb question, but my baby is 5 weeks old.  He eats every 2-3 hours on the dot on each boob for about 10-15 mins. If I pump one side and feed him on the other he is still hungry and if I feed him on both sides I have no milk after to pump. I would like to be able to pump a few bottles here and there so I can do things like run to the grocery store! Lol please help!