As adult Nyquil
contains hypnotics, its not advisable to be used in children.
You may use Acetaminophen
along with Tepid sponging for fever.
For cold and cough you may use simple antihistamines like Syrup Cetirizine
5 ml twice a day for 5 days.
Steam inhalation will help.
Give him plenty of fluids and bed rest
He will be fine.
Please do not use over the counter medications without discussion with your Doctor.
Hope my answer helps you.
Wish your son a speedy recovery.