I'd ask the doctor if he can have his heart checked for any possible hole between two chambers. Sometimes that can cause all kinds of infections in the body.
I think you have to put this question to your Doctor and tell him how concerned you are and would he do some tests .Maybe he needs his Immune system building up,may also be a good idea to ask if other children at his school get sick, as the germs are passed around , this has been a bad year for sickness . how about you get some supplements with anti oxidents,Vit C B complex also fish oil ..good luck