It's not just "what he can do", it's also if you will be able to ask him because riding isn't all about voice commands. Also, every horse can be taught different voice commands like for example for doing piaffe, passage, canter, slow down, speed up, extend etc. The most common simple ones are clucking to go faster/pick up pace/change pace and kissing noise for canter. There's also of course the "whoa" for stopping. Are you talking about trick training or something? If you ride a horse, you won't just be able to do a voice command and have the horse leap into a perfect passage. It's all about the riding, the voice commands just help communicate.