Yes, it's a coincidence. Type 1 is an autoimmune disease, meaning something (we don't know what, maybe a virus) caused his body to turn on itself and destroy the beta cells that make insulin. But the immune system itself shouldn't be affected, and certainly not just within months of his diagnosis!

Please don't take this the wrong way, but it's very important now that he's been diagnosed to make sure you don't treat him "delicately" or as a "sickly" kid. I'm sure he's really been sick, but don't fuss over him any more than necessary because it could cause him to make himself sick, you know what I mean? Make sure as soon as he's feeling better and not contagious that you get him back out there so he learns to live with this in a healthy way - he will have a long, happy life in front of him if he does.