How would you explain to boss having an anxiety disorder?

Cautiously: Unfortunately there remains a lot of stigma out there pertaining to mental disorders and unless your boss also has anxiety or or is knowledgeable in the area, i wouldn't much if anything at all. You run the risk of being viewed as unable to do the job, being passed over for promotions, to name but a few. Hopefully, you can get anx under control so it doesn't impact work and won't be an issue.