Aidyn went through this stage of very early rising around that age, and so did many of my friends little ones too...
I occasionally had success with blacking out the blinds/curtains (by hanging a dark sheet over the top).
Do you think it could be the cold waking him? As that is usually the coolest time of night/morning.

I think its fairly common... and I think its something that doesn't last forever. Aidyn's early waking periods usually lasted no more than a couple of months... Hopefully your DS's will be much more short lived!