He is young.
This is the season of colds and flu.
He is in school.
In school there are many types of viruses/colds that are going around.
Kids at this age, are hand-sy. They touch everything, and their face/mouth/eyes, they sneeze and cough onto others not using their elbow, they touch doorknobs/faucets/desks and other kids too. Hence, things contaminate others.
I know, I work at my kids' school and see this happen EVERYDAY.
Viruses, live on surfaces for days. Hence, it can be caught by others.
Until about 8 years old, kids can get sick often. Because of their habits and tendencies to be very handsy.
Per our Pediatrician, after about 8 years old, a child's tendency to get sick all the time, lessens. Because by that age, they are better about washing hands/coughing in their elbows/not being so touchy and are more hygienic.
So keep that in mind.
My son, who is 6, has gotten sick a lot, in Preschool and this year. He has been sick, back to back with various cold viruses, stomach illnesses and bronchitis etc. and has missed about 12 days of school, thus far.
I get a Doctor's note each time. He has not gotten his tonsils out. There is no need for that, with him. It will not stop him from getting sick or not.
And when he was younger, he had Croup and sinus issues/infections.
Once, he got sick back to back. It was not a relapse though. It was a different, virus... that had similar symptoms. Our Pediatrician, is also a specialist in contagious diseases etc.
I have Asthma. And am allergic to dust/pollen etc.
You should get an air purifier for your home or your son's room.
And do not vacuum... with him around or in the room. When vacuuming... it kicks up dust in that room, dust you can't even see.... and it can exacerbate his Asthma. So when vacuuming, keep him out of the room for about 1 hour. Until the dust settles down. And keep an air purifier on. Or have him wear a medical face mask.
Also, unseen by the naked eye, there is pollen/dust everyday that is blowing around outside in the air. And it can adhere to our clothing. So, when he gets home... have him change clothes, into something else.
And per weather/seasons... well, during certain seasons, the air quality is either worse or better.