DS3 is 17 months tomorrow. He has always been a pretty good sleeper, going through the night from a few months, self-settling, good with naps etc. He's had a few periods where he would wake in the middle of the night for long stretches of time (up to 4 hours 
), but these seemed to be linked to a burst eardrum & then a developmental leap.
His sleep now, however, has just gone really bad & I'm not sure why. He seems to get very tired in the evening & be ready for bed & some nights will settle OK, but then he'll wake up a couple of hours later. When he does it will be again for up to 4 hours & he just seems very wide awake & ready to play. He'll then get up at the usual time in the morning. If this doesn't happen, then some nights he just won't go to sleep in the first place, sometimes needing an extra couple of hours before he will go down.
We don't really know what to do. We've tried bringing him downstairs & watching TV or playing quietly. We've tried medicine, more milk, back rubbing, rocking, singing. We've tried a lavender body lotion applied before bedtime. The latest thing is that DH is sitting in the room with DS but not engaging with him at all, but he's been up there for about an hour now & it's still not worked 
Any ideas on what we can do? Is this another common sleep regression? Do we need to drop his nap - he'll usually have about 3 hours from 11.30-2.30ish? Please help!