Fair way to split rent -- roommate moving in earlier than me

[EverettKings]

Moving into a new place in SF. Roommate's job starts a month earlier than me, so he'll be living there without me for a solid month. We have a one-year lease.

What's a fair way to split rent for the time when he's there without me? Him paying double for a month doesn't really seem fair to him, but me paying a full half and not living there doesn't seem fair either.

OOT????