S.H.
He has a lot going on.
1) he is almost 18 months old. On the cusp of an age change and 18 months is a growth-spurt time.
2) he has been sick
3) he is teething
So ALL of these factors, DOES tweak a baby's sleep ability.
Even adults have a hard time sleeping when they are sick.
This is a temporary phase. Not permanent.
Kids this age, need about about 11+ hours of sleep. Plus naps.
IF you keep him up later... this is not the solution.
Because, being over-tired actually makes it HARDER for a baby or child, to sleep and fall asleep. And they wake more.
Keep his bedtime, the same. Don't vary it too much, or this will backfire.
Again, he has a lot of things going on.
And developmental changes too.
ALL of this, tweaks sleep.
It is developmental phases and due to his cold and teething.
If we adults had all of that, we'd have a hard time sleeping, too.
Do NOT skip his naps. Again, if you skip naps, this will only make him sleep deprived and he will have lack of sleep.
When he wakes at 4:00am... then he would need an EARLIER nap time.
Don't make him stay awake... and skip a nap until the afternoon.
And if he takes a morning nap and not an afternoon nap... then put him to bed at the same time at night.
The thing is, the more you fluctuate and vary his sleep/nap times, the worse it will get.
And go by his cues, per tiredness.
Also try give him teething remedies.
Teething wakes babies.
Plus he has a stuffy nose.
So try and remedy that.
My son at that age, napped twice a day, for 2-3 hours each. AND went to bed at night, about 7:30pm.
We tried to keep it the same, everyday. Because, otherwise it screws up their sleep.
Next: sleep is never a static thing in babies/kids. It does get tweaked, per developmental and cognitive changes. Sleep patterns, changes. Per a kids' age etc. and growing pains. But it is mostly a phase.