i did get call backs and used wiwo. What works wrt to asking me to stay with him even now is that I tell him I'll be back to check on him/ stay after doing xyz and he's usually out like a light within 5 mins...
There are times I've told DS I need the toilet and will come back to check on him, and yes he is usually asleep within a couple of mins.
Sara, sorry I'm not great at working out day lengths but I do know with mine I had to work out day length based on how much sleep he would have in 24 hrs. For us it was 2hr nap 10.5 hr night, then 1hr 45 nap and about 10 hr night so his max sleep was around 12hrs. Here then it was a 12 hr day when he dropped his nap but many do a 10.5hr day and a longer night.
I see you've mentioned developmental disturbances too, I would have thought that very likely with him just turning 3yo.
Good luck with the night!