Hi there, welcome to BW forums
I've never come across this before. If you don't want to give your LO formula it would be ok to just stop offering it and to continue to BF. Your body will produce the amount of milk he needs without you needing to measure how many ounces he takes.
You might initially feel like you're back tracking a bit as he may need to nurse a bit more to stimulate your milk to increase, but I'd expect this to be a short time. He might also just take those additional calories in solids when you drop the formula, at 9 months I wouldn't consider this a problem so long as he is having 4 good BF per day (I'd expect any LO this age to have 4 proper milk feeds plus 2-3 solids meals per day).
Of course, in other circumstances someone might have made the decision to mix feed with some BF and some formula, which is also ok, it's a personal choice how to feed your baby. It looks like you prefer not to use the formula though so I'd just drop it.