i wouldn't barge in on him. nor would i turn basic personal hygiene into a parental crime-and-punishment seesaw.
have a low key eye-contact conversation with him. not too lecture-y. 'marmaduke, it doesn't seem to me as if your showers are long enough for you to be cleaning yourself appropriately. you need to scrub your head with shampoo for a minute or two, rinse it well, and scrub your body all over, paying special attention to the smelly parts. you don't want to be the kid who's walking around stinky.'
if he emerges too quickly, let him dry off and get dressed and then give him the whiff test. if you honestly can't smell anything, let him roll. but if his head pongs (and it will if it's not shampooed) then quietly and without drama or threats send him back in again.
it'll get old quickly.
khairete
S.