J.L.
I would not have him wear the same clothes every day, not only is that gross (sweat and all) you are letting him start a bad habit.
I would show him that he puts his clothes in the hamper every night and then let him pick out tomorrow's clothes that night. As someone else said, it is a great habit for hte school year. Maybe go pick out some new clothes that he likes will make him want to wear them with his favorite characters, colors, etc.