I have much less experience training than you do, so take this as just a wild idea, ok?
maybe he needs to learn to have more and better respect for the bit first, before you work on him making transitions with his hind engaged. You know how they say ride the horse from back to front? But, if the front isn't "there" , then there's nothing to ride him TO.
So, maybe start getting him more responsive to the bit in just plain comeing to a halt, first, then coming to a halt and rocking back off the bit (soft in the poll and a tiny flex of the jaw to the "inside" ((whichever side you decide is the inside)), and then actually have him stop and back up a step or two.
Once he is more responsive to the bit, you shorten this to the place where he is not sure you are going to fully stop, or just half halt and then continue on.
Then, you start getting him thinking halt with seat and rein, when you feel him start thinking to rock back, you firm up your hands (the front) and ridethe back into that front.
One thing that helps with a horse that does not want to stop is this: once you feel the horse has got the message to stop, and he has started the process, give him a release in front BEFORE he actually stops , trusting that he will fnishe the process on his own , as if it's his own idea. I find this works well with Zulu, who sometimes doesn't want to stop.
If there is a big reaon to run, like other horses running in front, it will not work, of course.