L.A.
Oh my gosh, our daughter was always falling bumping crashing into things.. She was around so many kids who were daredevils.. I remember at day care she caried her friends "bloody tooth" (she called it a bloody toot) to the teacher..
I am sure your son will be fine. Kids are exposed to so many bodily fluids it would make your head spin if you stopped and realized.. .. Also not all of these fluids are noticeable.. Any public place has the chance for exposure.. I will not begin to list them because you may stroke out.. Use your imagination.
You asked so the Doctor is offering to test, but I am sure he is fine..
Kids are extremely resilient. He did not have an open wound, you keep him very healthy and active.
Do not go searching for possibilities, you want your son to grow into a non fearful active child. Let him explore, never hold him back.