Didn't read replies, sorry if I repeat something...
First, I'm glad that you concentrate on not interfering. A lot of times, riders are not aware that it is their lack of a stable position that is causing rails to drop. If you get up too early, weight is on the forehand. Left behind, the horse gets a catch in the mouth. Sitting down over a jump, the horse will sit on the back rail.
My advice? Oxers and grid work. It takes a month or two for grid work to be really effective. (I started gridwork today with Mav.)
Make sure that it's not you that is affecting his jump. My sister's horse is extremely careless. As soon as we widen the oxers, his attention is focused and he is more cautious of his back legs. Especially ascending oxers.
Really, you have the right idea. You can try open front jumping boots so there is still protection, and he can feel the rail when he knocks it.
Make sure that your distances are correct. Jumping deep can cause knocks with the front legs. Jumping long can cause back rails to fall.
Like I said, ascending oxers and gridwork. He'll come around :)