"What happens if we do nothing? If we do watchful waiting?"
"What happens if I don't take these meds?"
"How likely is that bad event? Tell me in terms of numbers per 10,000 people rather than percentages".
If he can't justify his decisions, then he's probably using instinct which is unreliable in the case of rare diseases, or he doesn't know how to diagnose known diseases which means he's incompetent.
Some people are offended when you challenge them for reasons for their advice, but I think that's because they aren't confident in it themselves or feel superior. Doctors shouldn't be in either of those positions and should be willing to tell you how they came to their conclusions.
Gerd Gigerenzer has a book explainin it better than I do.