@mshamdann- Welcome to Mayo Clinic Connect. I understand what you mean. Sometimes there are just niggling questions that need to be asked. I remember when my son was still a baby with a bad cough. It broke my heart. I am assuming that your son takes Chewable Montelukast. I believe that its brand name is Singulair. I have also taken it for allergies. I don't have asthma but sometimes I have needed something stronger.

I'm not a neurologist but most of the severe side effects that could happen are related to mood changes, such as depression and suicide. I know, I'm the bearer of bad news. But remember that not all children develop this. But it does bears watching.

Also, the FDA has issued warnings about this.
https://www.scarymommy.com/fda-black-box-warning-montelukast-singulair/
https://www.drugs.com/fda-alerts/1654-0.html
Has your son exhibited any signs of emotional upsets (I know that 3-year-olds can be difficult!)?.