Mixed feeding question

Jennifer

Hi all, my baby has been exclusively breast fed until now by breast and expressed bottle. I am wanting to introduce formula to mix feed. I have bubs his first bottle this morning and he only took 90ml approx. He is 9 weeks old and usually does anywhere from 90-130 ml of breast milk when expressed. Is this normal? Is he just adjusting to the formula. He took it well and hasn’t upset his tummy or anything. Thank you!!