The hill work is a great idea. On the flat, do long and low work, making him work from back to front.
Transitions are fantastic, too- especially downwards transitions. Walk-halt, trot-walk, trot-halt, canter-trot, and, later, when he's ready, canter-walk and canter-halt.
Edit- He's a handsome boy, he just needs weight and muscle- I hope you share your progress with us!