15 months old baby refuses to drink formula milk

Hi Everyone, good morning! I'm have been exclusively breastfeeding my baby since his is born. We had started solid food for him after his age allows. Breastmilk is his main milk even after we enrolled him in infant care center. However, recently, my milk supply very very low, only 20 -40ml per pump (like every 5hours) which not enough for his drink. As such, we have been trying to introduce and feed him different types or brands formula milks but he refused. Is anyone of you experience this kind of situation? How to solve? Please advise. Thanks!