S.H.
... the Nebulizer.... at night.... may very well be hindering his sleep.
I have Asthma... and nebulizers usually use Albuertol in it.... and Albuterol... can make one.... restless/hyper etc, and thus, going to sleep is hard.
For my Mom, who also takes a nebulizer... it affects her sleep... or falling asleep as well.
For me though, it does not.
Each person is different.
I would.. speak to the Doctor... and see if dosage or timing/time of day... for the treatments... can be adjusted....
But during the day, when he is NOT on his nebulizer... of course, he can sleep. And he is also probably very sleepy, during the daytime... because at night... his sleep is affected by the medication....
DO NOT cry it out.
He is so young and has health issues... LUNG issues. Crying it out... will NOT help that.
Don't worry, both my kids at that age as well... still had trouble falling asleep on their own. No Biggie. I did not see it as a big deal.
Usually... from about 2 years old.... a child will then mature.... enough... naturally, to have a better ability about it....
You have to... consider his condition/health/medication.... in such a young child and it interfering with his sleep... ability. He is not doing it on purpose...
all the best,
Susan