Well, your son could get sick, just exactly like he could get sick if he touches a doorknob, stair railing, kneels on the floor, or the like, and then touch his mouth. Or if food prepared in a restaurant is served by somebody who absentmindedly rubbed her nose, sprayed a bit while talking, or sneezed. We all get exposures like these all the time, and there's no particular reason to think that a cup lying on the ground was left there by a sicker person than those who share their microbes with us on a daily basis.
Recent research shows that children exposed to lots of germs and dirt (just the normal stuff) are generally much healthier by the time they enter grade school than children who grow up in unusually hygienic surroundings. AND the "germier" kids have significantly lower rates of asthma. Our immune systems actually need these exposures in order to train themselves to recognize nasty invaders. Current thinking is that when the immune cells don't have that work to organize their responses around, they begin to attack the body itself, resulting in auto-immune diseases.
And of course, exposures that result in colds and other illnesses also immunize us against future exposures to the same bug. You can get three colds in one season, but they will be caused by three distinct strains of virus (unless your immune system is compromised, of course).
So do your best not to panic – it won't keep your son well if you do. Just be alert for any unusual symptoms. None of us can promise you that he will "be fine," but numerically, his chances of picking up some terrible disease are probably not much higher than getting struck by lightening.
