Theres always a cure. I assume he is crossing from the knees down. That is just laziness. the easiest way to fix this is unmounted. Lets assume he is sensible and lunges fairly well. Set up a grid-gymnastic jumps placed in a straight line. They dont have to be higher than 18"-2'. You may have to adjust distance slightly. Cross rails encourage the horse to bring his front end up,oxers[fence with width]get them to reach and round up,verticles[height no width] rock back on thier haunches. Try settin up in a straight line. A cross rail 12ft to a cross rail oxer- 12ft to a ground pole- 24ft to a single x rail[put the x to one side-high side to leg that hangs lower] 12ft to a verticle 12ft to a xrail oxer. If your standards are low enough, do this off of a lunge at first since you say that he does trip. If he would happen to fall we don't want anyone to get hurt. You may have to ride him though looking at a grid straight on can be very discourageing as all you see are poles and no end and he may not even attempt it.