My daughter constantly got strep - I'm no doctor but I can tell you our experience. The doctor threatened that if she got one more case of it then he'd take her tonsils out. Wonder of wonders, she didn't get another case of it. No one in our family got it and we were definately exposed. Yes, we carry the "germ" or "bug" in us all...when our systems are down, the bugs go into action.
One thing I'd recommend....and this comes from having a son that had Menningiocci disease, get him on some meds or foods to boost his immune system. It sounds like he's "down" and just needs to be able to build up his immunity again.
BTW - my experience with this is over 30 yrs old...there is always a possibility the knowledge of this has changed...that doctor's will disagree with me. Lord knows they're always coming up with something new. DO talk to the doctor and do research online at WEBMD.com