ELEVATE!! When my boy stops using his hind end I pick him up and put him on it so he has to use it. When cantering think of getting your horse to hesiate on their hind legs before taking the next step with their front.
To know if your pony engage his hindquarters, you need to feel your horse pushing with his posteriors, that will lead to his back that rises and the head that is light, when he puts weight on his back, he will lower his hips and carry himself in order to propel you out of the saddle (especially on the trot).
If he doesn't engage, he will have his head up and will grow hollow his back, you feel it because your are less stable on your position.
To help him, you first need to be really straight to put the weight behind.
Then, you can work on lunge, it will be easier to see the posteriors engaging or ask someone to be around and tell you when he's engaging.
Work on full neck extensions, on a circle with bending, transitions within the gait or receding ones, make him go back or use every lateral movements for the hip (haunches-in, haunches-out, shoulder-in...)
Finally, you can work on ground poles to really feel your horse rising.
I hope i could help you :)
You may feel your being lifted up. Right now i am treating a mare that has kissing spine (dorsal spinous process impingment). The issue was created when not engaging the hind end.