Slowing them down is tough, especially ones like your boy, who has had two years,
not two months, of excessive speed. My girl had the advantage of not having it impressed on her mind as much as your walker does. (Speaking of, she is a part TWH -- the rest is NSH. Little coincidences.)
For the first rides, I allow nothing beyond a slow flat walk. If he speeds up, try the following:
If the area is open enough, I would pull him into a circle (relatively small -- 30 feet or so) and allowed him struggle. He can trot, pace, walk... His choice, but he wasn't getting anywhere. He hopefully will realize that all going fast will do is halt progress and make hard work for him , so he will slow to a normal walk . Once he is calmed down, allowed him to walk forwards along the trail as usual. The second he starts trying to take off again, repeat the circling.
In a perfect world, you would first do this exercise in an arena, so he gets the point before heading out onto the trail and already knows a little of what you're asking for.
The KEY is to be consistent.. Never let him get away with changing speeds without permission. The other most important thing is to never get mad. Go into the ride with patience, knowing what is about to happen isn't going to be fun for either of you. When circling, act like it's a nothing special section of trail. Let him figure it out without worrying about how mad his rider is.
I have several different slowing down methods. If this doesn't work, we can try another. It depends on the horse. What worked for my speed demon may not work with yours.
Your speedy will probably also take several weeks to "fix". Keep the faith! Nothing is impossible.