Based on the OPs first post and excepting the fact that one of the illnesses was pneumonia, it sounds as if he's just getting a lot of colds and flu-like illnesses.
My son seems to get a lot of these but I don't consider him sickly. He definitely could do better with handwashing and, due to lots of allergies, he's always got his hands on face, nose, eyes, etc. He gets too many colds in my opinion. Sort of sounds like the OP's son. The difference is that, unless he has uncontrollable snot or coughing, I force him to go to school. As another poster stated--I look at what he's got and question as to whether I would be expected to be at work and would my coworkers come to work with such a thing. Not always the best guide.

Anyway, most times it's "Get to school" unless there's a fever. If I let him stay home for everytime he had a sniffle, he MIGHT have this many absences.
It sort of sounds to me that the OP is not overly freaked out about his illnesses but, based on his history, is sort of "pampering" him when he gets a cold. We all have to use our own judgment on these things but I try not to do that so much because I don't want my kids to think that you can take off work or college or anything if you're not 100% feeling well.