Does he change clothes as soon as he gets home? It might be a good idea to wash what he's worn that day as soon as possible after he gets home. This includes socks.....shoes should be left outside or in the garage. Whenever my kids were in school I made them take off their shoes and leave them in the garage before they came in the house.
Well, we all left our shoes in the garage.....we just didn't wear shoes in the house. And if they were sick or had a URI I changed their pillowcase every night. If I had the energy I'd change the sheets, too. Might have been a little too much but they always seemed to recover quickly and the colds didn't get passed around too much. Oh, and all used Kleenex were flushed!