Try to find out what he has derived by doing such act.
It is human intention to get close to people by revealing confidential information and when it pertains to their performance and rating it is bound to be more though not intending to harm the organization and its culture,but may be to get the favors in out put or motivation etc.
Beyond that what action can you take on your boss, I suggest you be cautious while taking precautions for the next time. No doubt he might be at fault but what can you do now is the question. If needed (and you have access) report to his superior and get the things into control.
8th May 2018 From India, Hyderabad
The other thing is how valuable are you ? If you are really valuable the company will take some action. Else they may end up firing you for trying to take action
9th May 2018 From India, Mumbai