DON'T LEAN FORWARD. Sit tall and straight and as with a distinct and clear aid. Bring your inside hip forward, outside leg back and maybe even kiss (that is how I know a lot of horses are trained to canter, with the kiss that is). If he is still not listening with clear and concise aids, I would consider using a crop in your case. In using the crop you ask first and then if he doesn't respond, smack him behind your leg to get him to go.
Make sure you have a good position and your heels are down in case he tries to give a little buck at the crop (some horses/ponies do occasionally though usually nothing bad).