I am not sure if you have tried this yet. I would put him to bed and then if he wakes up do not go and get him unless he is really crying. Kids will change their sleep patterns every once and a while and it may take a couple of weeks to get them back on track.
I know when my son started this he would take a nap at 4 and sleep for about 2 - 3 hours. Then he would want to stay up until 10 or 11. I asked the DR and she said just let him take the naps and go to bed that late. She told me it would be hard for a couple of months, but he would slowly go back. Within 2 months he was back to his normal sleeping habits.
at 8:32 AM on Oct. 7, 2008