Bring a schooling whip. But I wouldn't give him loads of little taps, I would just give one big one at the start of the ride. Kick once, kick twice (big kicks) if he don't listen, give one big smake. I know it sounds mean, but it's nicer than all ways nagging him, and soon he'll learn to go forwards with the slightest of leg aids. Lots of schooling will help too. Make sure he isn't too unfit, that helps too.