My student is 18 but has no piano experience. He has some musical background so he knows what the notes are on the staff but, obviously, doesn't know how to transfer that to the keys. What should I do? I have an idea and plan but I'd love to get your advice.
Hey there, I teach piano too. 
Firstly, I would start teaching him the five first notes (DO to SOL, thumb to pinky) for both hands (physically show him each note, and then he would repeat what you played, etc). Show him which keys are where on the piano and when he is comfortable with the five notes, then continue onto LA and SI. Start with simple songs that only involve those five notes and then progress to the other notes. And it would help if you had a 'piano notes chart' for him…I'm pretty sure you can download it or get it somewhere easily..
Hope that helped!