Every night!
Sometimes he skips a weekend day but if he has a game or is going to a family event or gets a haircut he knows to shower.
Honestly, I know some kids are difficult, but I don't know how families find themselves in this situation. Brush your teeth, shower, eat food, go to bed...these are basic things that kids who are not clinically oppositional should usually do without major issue (maybe with some "get upstairs, it's time to get ready for bed" reminders, but they should do it).