I think your concerns are ligitimate and I think that most parents will tell you that their kid went through a similar "sick stage". Regardless of how many times he is sick...you have to be the advocate for him and so...if that means many Dr. Appointments and lots of meds later... so be it. It could be because your son's immune system isn't at its fullest when he contracts yet another cold/flu, etc...which can lead to ear infections or sinus unfections. Some children are more prone to getting ear & sinus infections when it comes to getting a cold...I think it's fairly common. So, don't worry about taking time off work, or calling your Dr. for apt after apt, etc. If the infections don't clear up completely they come back, and they could be worse if avoided. When you talk to your Dr. and if he finds an infection, make sure you get a stronger and longer medication. Cefzil, Zithromax are good antibiotics and if they put your sone on a longer than usual regiment, perhaps it will continue to treat the infection even after the last dose is taken.
Good luck. Don't be afraid to bug the Dr. If it were'nt for us worried parent's, they wouldn't be in such demand.
(Just a tip. A person will begin to feel the first signs of a cold 3 days after contracting the germ. So, as soon as you see signs in your son, (or feel them in yourself) count back 3 days and many times, you'll discover that you were around someone that was sick or had used a shopping cart, etc...in which you could have contracted your newfound illness. At least you'll have someone to blame in most cases.) Good luck. :)