I know this may sound daft but in the long run it may help him. When I was young I can always remember being ill right up to the age of 14-16. Any cold, virus or bug I seemed to get it. But now I hardly ever become ill. The last cold I had must of been 15 years ago and I can’t remember the last time I had a sickness bug, even when everybody around me is going down with one. I’m no doctor but I believe all those times I was ill when I was a youngster has built up my immune system.
Edit-
Obviously I’m not telling you to mtfu over things like taking him to hospital, illnesses like chicken pox and other fevers have to be taken seriously