At this age, you will need about 5 mL per nostril, to be done at least 3-5 times a day.
You should see improvement in his cough symptoms within 24 hours.
Also, be aware that during this time of COVID-19 and considering the cough, you need to stay home with your son for a period of 14 days.
If your son develops a fever, I would recommend that you contact 1-877-644-4545. Also, if the fever has been present for more than 3 days, you should see a doctor.