We basically replaced an ounce of breastmilk with an ounce of formula at a time. Like, if he's eating 4 oz at a time, one day the bottles would have 3 oz breastmilk 1 oz formula. If he tolerated that for another day, then we'd replace another oz the day after, etc. But if you want to keep breastfeeding and he's having trouble with keeping it down, talk to his pediatrician about an elimination diet first. Then you can narrow down what's causing problems if it's related to what you're eating.
@brentleysmomma16 How old is he and would you maybe try cutting out dairy (if you consume it) from your diet for example and see if it makes a difference? I know you of course just want to keep baby fed, but formula really isn't optimal compared to breast milk. I hope you find something that works!!