Use mineral salts deodorant - the spray kind. Have him spray his feet and let dry before putting shoes/socks on. Do this daily after showering - it works! Also spray the inside of his shoes and let dry. The salts are odorless and kill bacteria. Give it time to work - the odor will go away as he uses it.
I've not had foot odor since I've been using the stuff, and I wear flip flops in public all the time now. No odor whatsoever.