I'm going to go against the grain here and say don't use your legs as these are the things you use to make the horse go faster. If you grip with your legs you are tense and if you are tense you will bounce around on a fast walk like a rock. Rather, you should let your legs hang down but do not allow them to flap about. Open your chest and sit deep- think of sitting IN the horse rather than ON the horse. Let his movement flow through your seat. Once you tap into his movement, try to slow him down. Lean back gently and grip slightly with your core (abdominal muscles) to slow him, you will probably need to use the reins. With the reins, use a half second to one second long pull to slow him down; a jerk will make him tense and he can ignore and lean against a steady pull. Chances are he is walking quickly because he is hollow or leaning on the forehand, can't really tell from the given description. If he's hollow, he is tense. He needs to have his neck in long and low position and relax his back. If he's leaning on the forehand, he is trying to counteract his forward lean by rushing forward. A gentle bump up on the reins is a temporary fix, learning to move his shoulders around with a turn on the haunches is better.
"The trouble with the world is that the stupid are certain and the intelligent are full of doubt"