The boss is being courteous in giving you the choice of when and how to announce this. If I were you, I'd appreciate that rather than complain about it.

If you prefer to have your boss announce this, ask him to do so.

But really, it shouldn't be a big deal either way. This is a fairly common occurrence. And it's going to become public knowledge anyway as soon as you start briefing whoever will be taking over your duties.
<!---->