This time of year when it starts being light in the mornings little ones can wake up, see the light and decide it is time to get up. Make sure the room has lightproof linings to the curtains. If you already have them, take child out into the fresh air during the day and wear it out utterly...also, is he still having an afternoon nap? If so, cut it out- he'll be vile and whiny for the first few days but after that he'll sleep longer at night. The clocks will go forward soon, so 5am will become 6am and bedtime will go forward too, try perhaps having upstairs  play and read quietly time, then bedtime 30mins later than it is now, might push waking forward half an hour or so and when the clocks change you'll be at 6.30, which isn't so bad- he is sleeping for 9/10 hours without waking and that is a good night's sleep ... one does just have to go with the flow a bit and go to bed early to keep up, it's annoying as you feel you have no life of your own, but you might have to do it for a while to catch up on your sleep too...the idea of a clock so the toddler knows when it is ok to come in and wake you and meanwhile read/play quietly is a good one, just don't leave felt tip pens where the child can get to them- trust me, a toddler coloured completely turquoise beneath its pyjamas is no fun....They do go through phases of not sleeping so much, just like they go through phases of not eating so much, once the days are warmer and he can get out and about and do some running, he'll sleep like a log for more hours...good luck!