When he changes in front but not behind that's generally a sign of not using his hind end properly, whether that comes from lack of balance or strength, being ridden from front to back, soundness, etc. A horse that has learned to just change in the front is very very difficult to fix. It's easier to start with a clean slate. If you can feel that your horse is only going to change in the front I would stop him and just do a simple change.
Some horses naturally get changes faster then others. I'm currently working with a horse that I was told "can't do a lead change". I'm starting from the beginning with him, getting his foundation a little better. You ride each horse from the back to front so it's crucial to be able to maneuver his hind end and get him using it properly. I started with turns on the forehand, easiest way to teach a horse to move his back around. Next came turns on the forehand and then leg yields. Horse needs to be able to move away from your leg and move laterally. Then came shoulder in/fore and haunches in to teach balance, strength, agility, etc. Then we started on a little counter canter. That was crucial for this horse, his canter just wasn't very good. All the while, we worked on getting his transitions very light and easy. I find that a horse must at least be able to canter/trot/canter with as little trot steps as possible, or really, canter/halt/canter. This horse was a bit of a freight train with transitions and wasn't able to do any of that. I haven't even started the changes with him, I'm just setting him up so that when we start he will physically be able to do it. And there's a much better chance of getting a clean change out of him that way.
Being a jumper I don't know what your views of dressage are, but my opinion is regardless of what you do every horse that jumps (hunter, jumper, whatever) needs to be able to do what I just listed above. I often don't think of it as "dressage", I usually just call it "good flatwork".
sidenote: Being 20 years old, I wonder if there's some sort of stiffness there that's gong to make it harder for him. Have you thought about injections for him?