Eventer, have you tried working with him at liberty in the round pen? I like to keep it low key and ask for a walk only for the firt 4 or 5 days. I don't use a whip but a thin willow which will whoosh the air nicely if you need to add a little pressure. Take a chair, stool with you and set it up slightly off center. Have him circle a few times then go sit down and ignore him. Read something. He needs to figure out how much he can trust you. If he doesn't approach after a while, get him walking again, then sit down. You are building a mental connection, not a physical one with halter and line. After a few days approach with your saddle pad. If he walks off, set it down and ask him to walk a few rounds then turn your back to him. You are allowing him a say. When he approaches, set the pad on again. If he remains, remove it and walk away a few feet. Wait a few seconds then approach again with the pad. If he walks off, leave the pad on him. Then ask him to walk. Same with the saddle. I'll leave the horse to walk around or stand while I go sit, to let him sort it out. I'll do this daily for 5 days. The repitition does wonders. When it comes time to mount, barely put weight in the stirrup. Let him walk off, then same routine as the pad. Continue putting weight in then switch sides. When he stands still, unsaddle him and put him away. Do this for a few days then use a mounting block and start leaning over him. I think you get the idea. When he remains standing so you can mount and dismount, he thinks it's his idea without you making him do so. Horses, knowing they have the option to leave will often stay. If you have time, just sit in the pen and read with some hay nearby. Don't touch him or talk to him, just read and ignore him. Every notice if you were fixing fence every horse would be there to help, but walk in the field to catch one and they all take off.