formula feeding struggle

HT
My baby is 5 months old and from the very beginning i have difficulty in my milk supply at one point we were told the baby isnt feeding well so i have to express feed him to monitor is intake, so considering atleast he will be getting the best i started expressing and then we were advised to give him a top up from formula because i wasnt producing enough.... Anyways curting everything short i am still low oan my supply i just make 14 to 18 oz a day and now since the last week my baby is not drinking wven a single drop of formula,,,, what shall i do now my baby is starving?