Funnily enough we recently moved him to a single bed (from a queen-size mattress - gave up on the cot a loooong time ago) and he's been doing ok. We're actually out at my parents' place for the moment where he sleeps in a queen size bed...when I'm lucky I get to sleep in the other room.

If he's moving around lots the cot might not work too well - DS hated hitting the sides.