recurring/lingering strep throat in 20 month old?

my son will be 2 in july. He has had strep throat at least 3-4 times since christmas. He never has any symptoms, but I have taken him in to be tested because of exposure (siblings and cousins). The last time was about 3 weeks ago, and he developed a rash. Since strep occasionally comes with a rash I didn't think anything of it. I took him in last friday because the rash had not gone away. I thought it was not related to the strep, since it was still lingering but becasue it showed up at the same time, the ped. did an overnight strep. they just called me back this morning and said that yes he has strep and needs to go on antibiotics. The nurse that I spoke with is not a nurse that he has ever seen, its sunday so she was just the one attached to the doctor on call, so she couldn't really answer any questions that I had. She did say that it was unusual for someone this young to have strep in the first place, let alone this much. I dont know what to do. why is he repeatedly getting strep, or, has he just had it all along? grrrrrrrrrrrrrrrrrrr.