I get 4 oz (both sides) from pumping 25 mins :(

When my baby eats he used to fall asleep eating one side and wake up to eat the other. These days he falls asleep on one side and wont eat the other when he wakes up nor will he switch sides when eating. So hes not getting what he used to get. How can i get him to switch sides before he sleeps completely or eat when he wakes up again?

He usually eats every four hours or so.

Please dont ask "how do u know hes not getting enough" or "babies get more out of u than a pump"etc. he also refuses bottles. I just want advice on how to get baby to relatch or continue eating.