You can also do a lot of stuff at the walk that will help your horse to pay better attention to you. Do forehand turns and haunch turns. Walk in a square doing haunch turns at each corner and stop and back in the middle of each side. Do A LOT of transitions. And when you are trotting and you start to feel him speeding up do a 10 meter circle, don't pull on his mouth. This gets your horse really paying attention to you because he's not sure what you're going to ask next. It is also helping to get him off the forehand and collected.
Another thing you can do, start at the walk, give him a loose rein and then squeeze with your knees, push your belly out as hard as you can (using what's called you "bear down muscles") push your seat bones down and back and lean slightly forward. Your body will scream at you but stick with it and you will feel him slow and eventually stop.