HELP!

I have a nudist on my hands and I don't know how to get him to keep clothes on!  Almost every-time I turn around my 3 year old has on no clothes.  Just a few minutes ago I caught him on the trampoline jumping , NAKED!  I called to him and  reprimanded him for not having on his clothes to which he said to me, "My clothes are outside. " he had brought them outside with him but failed to put them on his body.  I then said to him, "You can't run around naked"  and he responded  "I am not!  I was walking around.  Running around is not the same as walking around."  I couldn't keep a straight face to continue reprimanding him.  At the moment he has on clothes, but HELP!  I need some suggestions on this one.