You said the bumps don't itch- do they hurt? Are his knees rough? I'm going to guess that at 7, he's probably playing with his knees on the ground a lot- it could be caused from that, dry skin (even if they don't itch), or something along those lines. It doesn't hurt to keep an eye on them and if they change or are starting to bother him, have them looked at- other wise, just mention them at his next doctor appointment to have them take a quick look.