I think it's safe to say juice is traditionally not considered a medical liquid. In that situation, he is asking for an exception. It is his requirement to justify the request that his juice should be considered a medial liquid. It's unreasonable to say that he shouldn't have to justify its reclassification as a medical liquid without a few facts and disclosing a few details of his situation. Otherwise, every person would claim their soda/juice is a medical liquid and say they don't have answer questions because it is a medical thing.
I might agree, if juice were a regulated substance, and if people were required to walk around with documentation about their medical issues.
As far as reasonability - he's correct in noting that the TSA is not entitled to your medical history. Moreover, TSA screeners are generally not doctors (some may be), they're not YOUR doctor, and they're certainly not in a position to decide what is or isn't medically necessary for you.
Does this mean that you should be able to get your juice past security by claiming to have a medical condition? No.
It means you should be able to do that without having to disclose anything about your health, and the only remedy that should be available is for the TSA to screen the liquid if they feel it is necessary to ensure passenger safety.
Is that unfair? Maybe, but so is a ridiculous ban on a liter bottle of safe substances.
