persistent early waking - help!

[Tillysmummy]

morning (or is it afternoon now?) mumsnetters. I haven't been on here for 2 years plus so it's nice to be back
My ds (19 months) has never been the best sleeper (took him until 9 months to go through the night) but has been sleeping through now for about a year. He always used to wake at an ok time (6.45 - 7.30 ish) and it was dd (4 years) who has always been the early waker (6am from 1 day to 3 years!) but she has started sleeping a bit later and is now at school 5 days a week and really needs her sleep. However ds of course is now waking early. He has all teeth except last four molars (two top and two bottom). He seems to have been teething with these ones for about 3 months. He has been waking sometime between 5.20am and 6.00am for nearly 10 weeks now and I am getting to the end of my tether! .We really don't know what to do. We don't get him up until 6am whatever time he wakes. He stays in his cot and more often than not screams until 6. We then go in all happy and give him a nice cuddly morning greeting. This controlled crying seems to work after a couple of days and he then wakes at 6 for one day (massive lie in!) and then he reverts back to 5.30 again a day later. I understand he is teething badly and have tried giving him Calpol / Nurofen before bed etc but nothing seems to work. He still sleeps for 2 hours in the middle of the day and I wondered about cutting this down / out? He is still only 18 months now and expends so much energy all day I can't believe he doesn't need a sleep. In fact, today for example, he woke at 4.45 and so by 11 will be shattered and in desperate need of a nap. Also the other day he slept for two hours and it was one of the mornings he slept until 6. I know it sounds precious but I just can't cope wtih before 6am. 6am is bad enough but 5 something is just not on. DD is on half term at the moment but goes back to school in a week or so. I don't know how she will keep coping with the 5.30 start because it always wakes her and she just seems to be getting more and more tired. We have tried bringing him in with us when he wakes but unlike her (she used to go back to sleep ) he doesn't sleep and also I don't think it's a good idea to get him into this habit (and give him more reason to wake).

Any suggestions desperately welcome ....