Hmmm, is he learning lots of new stuff maybe? Language skills, something like that, that he's up thinking about?
DS does this ocasionally. He often wakes in the night, but generally goes back to sleep fairly quickly. Sometimes he's up for a while. He seems to want to go to sleep, but just can't manage it - I really have no idea what it's about. I hope he doesn't start doing it more often! It is sooooo nice to finally get some nights of good sleep, isn't it?