You can try pumping about 1h after he eats and start storing up that way. For me, I started saving up when Dh would give DD a bottle with formula (for whatever reason or another, it happened from time to time before I went back to work), and I would pump and freeze it. Then, I took out the milk a couple days before I went back to work so it could thaw in the fridge, and now I pump every day for her next day's meals.
Another thing you could do--and this is for later on once he's not a teeny newborn and eating 24/7--you can pump during a feed that he no longer takes. For example, if you don't do a DF, go ahead and pump at that time and store it away.
HTH a bit...perhaps others will share some insight, too.