How can I help my horse to lose weight, and fast?

Just like a person loses weight: diet and exercise. Keep up the groundwork. Slowly increase the intensity or the exercises and the length of the workouts. Since he isn't having any grain, you can't remove that. Try a nibblenet for his hay. It has tiny holes so it takes him longer to eat. I also suggest a grazing muzzle for when he is outside. My mare has a muzzle on from the first grass well into the fall, even when she is on hay. It slows her down and really helps cut down on her weight.