I know this behaviour to be common, though is not widely talked about. Ask him if he wants to make this a goal for himself, but do not start with the full bath or shower. Start small and build it up. ie, he has to brush teeth everyday for three days, then brush teeth and wash face for three days, then brush teeth, wash face, wash under arms and grion area for three days, and keep building on it.
Make the order any order, but it is too big a change to expect the whole lot to be done at once in his eyes.