I have Asthma and have had it since I was a child.
Asthma.... does not go away and it will always... recur.
Keep that in mind.
So, in addition to getting colds or other illnesses... Asthma is STILL always present. It is a condition that a person always, has. It is not situational. Except that certain "situations" can exacerbate, it and its severity or attacks.
There is no way... to predict, when Asthma will arise in a severe form or mild form. And what things, makes it arise or not. You cannot predict that. But you can make the home environment, more suitable. ie: using air purifiers and filters. Keeping no carpet in the house because it harbors dust. Keeping pets outside. etc. Keeping him away from people who are sick. Because, anytime an Asthmatic gets sick, their Asthma will get worse. And get the Flu shots. I do.
Keep in mind, that if sick with any cold, Asthma gets triggered. AND, it can also develop into other lung issues. ie: I once was sick with a common cold. My Asthma got pronounced. I later got PNEUMONIA.
So, you really have to learn about Asthma and its symptoms in children (because children often present different symptoms than an Adult would) and learn about it.
Asthma is very serious. People die from it. If there is lack of oxygen in the body and blood stream, that is not good. An Asthmatic, has a hard time breathing, because the bronchial airways... gets inflamed or constricted. Hence, the lungs cannot expand or expel enough air/oxygen.
ANY time, an Asthmatic is sick or has a cold... the Asthma... gets or can be, present MORE so.
Asthma, is a DAILY thing and daily condition.
With which, "maintenance" medication(s) has to be used. ie: inhalers. There are emergency inhalers and there are daily use for maintenance type inhalers.
Your son should be seeing a Pulmonologist.
It is not a matter if it is Asthma or not... because Asthma is ALWAYS present... even if you do not 'see' symptoms.
And a person has to take care, in light of their inherent Asthma.
As I said, I have had Asthma almost all of my life.
I have never, used a humidifier.
Cold or warm, humidifiers can harbor mildew.