My daughter picked up hand, foot and mouth disease at nursery when she was about 18 months old. It is highly contagious, and spreads very quickly. I'm thinking that if your son isn't showing any symptoms by now, then he's not picked it up the disease.
My daughter was fine with it, apart from the few itches from the rashes on her ankles and in between her fingers. CF-wise, there were no problems at all.