"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.
As the joke goes, "what do you call the guy that graduated last in his class at medical school? Doctor."
Doctor's aren't machines, they're more like mechanics, they see a symptom, use their knowledge to make an educated guess essentially, and then work from there. If their knowledge is soft it's likely an incorrect diagnoses could just be exacerbated by incorrect treatment.
Get another opinion, and then another, the next problem is deciding on siding with consensus or outlier.
A string of anecdotes of "I double-checked and it turned out the doctor was wrong" remains less than a trickle compared to the flood of patients doctors have to deal with, and a tiny fraction of the hypochondriacs they must deal with. Our medical system is broken enough without adding doubts on the competence of its medical professionals.
By all means, people should double-check what the doctors say, but realize the much worse consequences of sowing doubt.