i have to add one thing here. humans were not meant to live in highly sterile conditions. there is such a thing as too much sterility. i say let your son out to play in the grass and mud, let him get dirty and grungy. it is well known that exposure like this can create immunity to certain diseases in children. the elements in the dirt kick in immune responses. kids need to be in and on the earth and soil, its an important part of growing up. you sound like you might be overprotective (maybe? in more ways than one?) let him get out there and enjoy the grass and trees. if its winter, let him go sledding and make a snowman.
i know some disagree, but life is dirty, you have to learn how to deal with it.