Well for me an important point was to always keep your front hand (I'm regular so my left hand) straight - a lot of the time it'll lean back or forward which really puts you off your turns.
I'm not sure if there's any really good teacher but experience - he's going to have to learn to flow with the board and its counter-intuitive feeling. Gravity is your friend, not your enemy, and when he stops fearing falling over he's going to do it a lot less.
It also depends how good he is - it took me about four days or so to learn turns, and then honestly I only felt like I doing them well by my 10th day or so, and I've only done like 15, if that.
Also, "teaching" him, not "learning" him.