Horses that hop up through transitions are evading having to use themselves to push forward. So to them, it is easier to hop up into the transition, instead of moving [b]forward through the transition. Make sense?
I would take a few steps back, and work on getting him to push forward from the halt to the walk. If he accepts a dressage whip, grab one. At the halt, ask him to move forward by gently tapping his behind with the whip. You want to get the sensation he steps underneath himself and pushes himself forward, rather than pulling himself forward with the front end. When he takes a step, pat him, and stop him again. Don't stop too soon, as you would be contradicting the exercise by immediately taking the forward away. Repeat until you touch him w/ your legs, and he's already offering to push forward without the aid of the whip. Once you feel he's got that down pat, then you can begin to work on transitions within the walk. At the walk, ask for a more crisp walk. If he hops up instead of walking faster, take one rein and slightly
turn his head until he walks. Refrain from tight circles, as it reinforces his want to not moving forward. Taking one rein and quietly breaking his want to trot instead of using himself will break his pattern in a non-aggressive and non-confusing way.
Once you are able to walk slow, then walk fast, then walk slow, then walk fast smoothly, you can move up to asking for the trot. Use the whip if you can, let your hands go forward and tapp his behind into the trot and feel if it improves. If you feel the hop up as you are asking, just continue to tap his behind until he makes an effort to move away from it, which will be forward. :) Once he does, reward by allowing him to walk, then repeat over and over.
Simply put, hopping up is an evasion to going forward. So go back and work on the steps leading up to the trot and forget about the actual walk-trot transitions until he re-learns how to propel himself forward in the correct manner.
Good luck! Be patient! :)