What do I have to do, to get my 15 year old to wash his hands, after going to the bathroom?

Be blunt. In a nutshell, when common sense fails, let him know he will end up eating poop if he touches anything that he might eat, or if he puts his hands to his mouth. All of this can make him and other people ill. If this does not inspire him..
Mention girls. I always remind my teenagers that girls have almost superhuman senses of smell and perception, and walking around stinky for any reason will trip their radar. If he insists on doing that, all the girls in his immediate life will know he hasn't washed his hands, and i'm sure he doesn't want that. There may be an overall emotional immaturity issue for him to deal with also.