Pull the start relay and see if it quits running the starter when you pull the lever. If this drawing is correct I would disconnect connector 5 and see if you have continuity between the output of the Brake switch and the output of the start button. If you don't find it there check from 5B to 1B, then on 1A.

 

I rode behind a newb who repeated tapped his starter button while riding- then I heard him yell to the guy next to him "hey,my horn doesn't work".

 

take the housing to the starter button off and make sure there are no bare wires

 

Seems to be a "15K mile" thing.