Idiopathic means we looked at the things we know how to diagnose and it wasn’t those. The exclusion process is an elimination process.

Test for all the plausible causes, and if there’s nothing left you shrug and call it idiopathic. So either he was tested, or the doctors thought his symptoms were very atypical and didn’t run the test.

(I know two people and a pet with idiopathic something or other. In the cat’s case, the treatment was the same as the least invasive treatments for the common cause. The humans are just SOL.)