Start by petting him and talking calmly. Slowly put a blanket on his back so he gets used to the feeling. Then in a couple days you will be able to strap on a saddle. Lead him into some water that is deep enough to cover half his belly. Then hop up and HANG ON! The water will keep him from jumping too hard and throwing you. And it will wear him out faster.
Once your monitor is well broke in you will enjoy riding more. And you can even let your friends ride.