Somewhere along the line someone has screwed up a little with this horse. They've let it get away with not moving or applied leg pressure and not removed it when the horse moved forward, or they've started trotting and then jerked on reins/pulled him up a lot.
So I think from here two things are important - gradual pressure and release of pressure. You have to apply your pressure increasingly until you get the result you want everytime, and as soon as you get it you have to release. So if you ask for a trot as soon as you get it release, don't keep pushing for round or impulsion or faster. That can come later when he learns that legs mean forward.
Although, before I start ridden I'd do some work on the ground. He should yield to pressure in all ways so I'd teach him to move off immediately at a walk and trot in a roundyard, or perhaps on a long lead. It can be easy to increase pressure on the ground, but it's still important to use gradual pressure and then stop it all when he does what you want. If he learns the voice commands as well, then he'll understand what you want under saddle.