All you should have to do is put it in the opening of the mailbox so the mailman sees it. He'll redeliver it.
In general, one is supposed to be able to mail mail this way, not just stuff that was misdelivered. It's one of their services.
I might not with something critical, for fear he'll forget to deposit it when he gets back to the office, but one could ask if he gets much mail this way and if it's routine to to check his bag for outgoing mail he's gotten.
Remember this if and when you're too old to out. ;-)