S.S.
oooh poor boy!!
with 4 boys? I can tell you how fast things spread if you don't get on top of it.
Change the tooth brush.
Change the towels in the bathroom. Wash them in hot water. Dry in HOT temp.
Wash the pillow or replace it. I know it's expensive to replace but the germs are still there.
Clorox or disinfect (Lysol, whatever is a disinfectant - lemon juice works well too) all door knobs, toilets, sinks, anything he's touched. I put on a pair of gloves and just start scrubbing when the first one gets sick. When it moves through a family of six? It's just not fun.