Breast milk jaundice

Ca

My 4th and last baby and I have been dealing with this. I've never had a baby with this before. Anyways his bilirubin has not improved in 9 days. He is currently 3 weeks old. It has been recommend to substitute with formula to help his numbers. So my question is how much formula does a 3 week old infant drink? I need to mix it with breast milk and hopefully will only be for 5 days or so. Any help would be appreciated.