Talk to him, walk behind him, enter stealth, wait for the eye icon to empty, and steal the medallion. When I did it I had a 9% chance of getting caught, but he didn't notice.
As long as you're behind someone, and outside the range of the pickpocket prompt, they'll lose track of you after a few seconds. --- "I'd rather betray the world than let the world betray me." -Cao Cao