I believe you would have to mix up the formula first then add in however much breastmilk you want. Start gradually, so maybe like 1 oz formula and 2 oz breastmilk or something like that.
Enfamil is closest to breastmilk. It’s inspired by it. You can do half formula half breastmilk until he’s used to it. And do less and less breastmilk to wean him.