That would be my guess but he stayed home (quarantined) with his parents. Our daughter kept bringing them whatever they needed and leaving it in their driveway/garage.It is quite strange - although kids are commonly show no symptoms, they usually test positive with the rest of the adults. Their lack of symptoms makes many believe they don't have Covid so they act as superspreaders from home-to-school and school-to-home. Notice the huge peak of new cases start in mid-August/early September.
Think twice about blaming the surge of cases on other things. It's not the cooler weather - we had unbelievablely warm weather in September snd October this year, most states reopened well before that surge, we were eating dinner outdoors comfortably until the last week of October in western Pennsylvania.
Again, your grandson was either lucky or had a false negative test result.
Ron