LisaG covered it pretty well. Have you ground driven this horse yet? If you haven't that's going to help teach that forward motion with out leading. If you don't have a surcingle for ground driving a way you can substitute which this is what I have done before I had a one is I would saddle the horse I made myself driving reins with long rope that I attached clips so I could attach it to the bit and I ran those ropes through the loops of my breast collar rings on my saddle and ground drove that way. It was my poor country girl substitute but it did it's job
. You'll need a lunge whip as well to urge the horse forward it takes so practice but once the horse knows what you want you can start working on the forward movements and your turns. You can also do some lunging with this horse and work on respect and voice cues for speed control and work on his whoa. You can probably start sitting on this horse from the sounds of it. Start off with putting some weight in the stirrups if he's relaxed lay across your saddle that way if he gets spooked you can make a quick escape. If he's pretty good have someone hold him and sit on him maybe lead you around a bit he'll probably be fine if especially since this has happened before. When you start riding him without a leader I would just make sure to give him lots of praise. When you work on turning him even if he takes one step release pressure and give lots of praise. Keep your riding lessons short work on him walking and turning at first and build from there. From the sounds of it you've been putting a pretty good foundation on him. If you haven't yet work on him yielding his fore and hind quarters mine all do this on a touch. Do you have any specific things your unsure about. It's hard to think of everything to do off the cuff lol.