Some reading about dressage will be helpful. I am not sure which book I would most recommend, but you need to read about the training scale, and start with the excersizes that are at the bottom. ANd, as said, they deal with getting the horse to move forward freely and rythmically, and to learn to reach down and forward. If you have never done this before, could a dressage teacher teach you how to achieve this on a horse that is more cooperative?
You , yourself, must be able to have a fairly independent seat becasue you will not be able to hold the reins tight and use them as a "crutch" to stay on.
Can you ride him, say in a round pen, trotting without any real rein contact? letting him put his head where he will? Start there, making sure you are not impeding him. Then start with taking up an inviting contact and getting him to stretch downward.
Read up in book or internet "riding long and low".