Talk to your boss to clarify if you are his mentor. If yes, the three of you sit down and let the boss anounce the decision. Then you can start manage his work. If you don't weigh anything in his annual performance review, you mean nothing to him no matter what the boss said to you in private. However, it doesn't seems that the boss really want you to manage him. Then just forget about it. Treat the new guy as the same level. Maybe you'll get a new friend.