Let's separate two things: The way I believe the law is right now, and the way i wish the law was right now.
You are conflating both. Like most lawyers, I can be dispassionate in my analysis of things, regardless of my personal feelings.
I actually have a very good understanding of what the law says right now. I personally think it's become seriously perverted from its original purpose, but that does not change how the law has developed, it only means we need to fix that.
As it stands right now, the government very likely has the right to demand intimate medical information from people when that intimate medical information is the reason they are citing to avoid legally enacted security rules. Again, i think this is mostly bullshit, and shouldn't be that way, but it doesn't change that it is. The supreme court and others have been very protective of border and transportation searches made for the purpose of security.
Note, btw almost any ADA claim he makes would fail
The ADA does not actually prohibit these inquiries on it's face (it would if they were made for a job related purpose), and aircraft is specifically exempted from most of the ADA's transportation requirements:
"The term "specified public transportation" means transportation by bus, rail, or any other conveyance (other than by aircraft) that provides the general public with general or special service (including charter service) on a regular and continuing basis."
If your hope is to get a court to decide that he won't have to justify it's medically necessary liquid, there is a roughly 10% chance that will happen (maybe less).
If your hope is for a court to decide the TSA needs actual individualized suspicion before fucking groping people, there's an even less percent chance of that happening. In fact, courts have already held exactly the opposite numerous times (due to them being held to be administrative searches)
Your best outcome here is probably that they'll require the TSA offer to ask this stuff in private, and be required to guarantee confidentiality.
At least right now, no court is likely to stop them from doing it.