Baiyu sooooo sorry to hear about the sleep problems. No ideas, past recommending the Healthy Sleep Habits Happy Child book and watching the clock, once he's been up for 90 mins start a wind down routine for a nap, whether he seems tired or not.
Watching for tired signs (decreased activity, slower motions, less vocal, quieter, calmer, disinterested in surroundings, eyes less focused or drooping, yawning)
rather than waiting for over tired signs (fussing, rubbing eyes, irritable, grumpy)
I found I was waiting for the second section and by then it was too late really and he'd go bonkers as he was too tired. Now, even the hint of a yawn I put him in his cot and he usually goes to sleep within 15 mins but quite calmly.
Please ignore if you've tried all of this....just thought it might help. xxx