Increase milk supply/ mix formula into breastmilk

I have a 6 day, I am a FTM. He is so sleepy that sometimes he will be on the breast for ever just on and off eating. This doesn’t completely fill him or empty me so I’m pumping after “feedings” (more like snacking…).

I’m pumping for 20 minutes and get 2oz in total per pump session. How could I increase supply? This little guy really gets cranky if he isn’t fully satisfied and I read online about mixing in a bit of formula with breastmilk to fill the gap— has anyone tried this?