Great post by upnover.
A horse that has learned to just change in the front is very very difficult to fix.
So true, and so poorly understood. I usually counsel people not to attempt to teach changes until the horse actually offers one. For instance, on a green horse; if you've been doing simple changes in the corners on courses, a day will come when he offers a change on his own before you ask for the simple. That's the time to start schooling them; asking for them before then usually leads to trouble.
Anything you can do to improve his overall fitness and conditioning will help. Walk and trot work, shortening and lengthening on hills builds the kind of muscle that support clean changes.
But, a horse that will execute a clean change cantering over a pole but not on flat ground is telling you something, and I think it's that he is in some way stiff and uncomfortable. If you *make* him round and lift his hind end with the pole, he'll change, but he' just prefer not to have to work that hard, most likely because he hurts.
So I think looking into hock injections isn't a bad idea at all. I'd also have a conversation with your farrier, and if he isn't already on a joint supplement, I'd consider doing that.
In other words, I don't think this is a training problem, I think it's a physical problem. And I can't bring myself to be too critical of a 20 year old horse showing in the jumpers getting around course the best way he can.