If he's full after one side and a tiny bit from the other, then that's all he needs! As long as you start on the side he ate less on, you should be fine. You don't need to pump that side after a feed unless you want to store the milk or you're trying to increase your supply. Actually the same goes for pumping the side he just drained - if you pump after he eats, then it tells your breast to make that much more the next time. So again, if you're not storing it and you don't have concerns about low supply, then you don't need to pump it.