We're on a mixed feeding regimen. Every 3 hours, 40 ml formula and bre…

We're on a mixed feeding regimen. Every 3 hours, 40 ml formula and breastfeed on demand. The son is gaining weight well, I want to switch only to breastfeeding. Who has done this? How best to do it? Should we gradually skip one feeding of formula? Or can we immediately just offer only breast milk? Thank you for your answer.