DS always had one big sleep and two little sleeps at your DS' age. The time of the big sleep varied depending on the day, I just let him do his own thing. I think he mainly had his big sleep in the morning though.
That being said, if I needed to get somewhere I would wake him up. I'd do this by simply going into the room and just making a bit of noise, usually by opening drawers etc. He'd start stirring and kinda wake up on his own if that makes sense.