If you ride in the arena go on a 15m circle and ride many transitions between canter and trot or canter and walk. the transitions give him the power to carry himself on the Hind leg. I guess he is pretty fast in canter ?
What also could help is minimizing the circle for a few rounds like a helix and then maximize it in the same way.
And i think what Leigh hunter said was also Right. Maybe you try both ways because they don't disturb eachother. It is both about achieving more self carriage and more balance.
A lot of canter work on the lunge really helped my Hackney cross. They learn quickly to balance themselves and canter slowly, which builds the right muscles and improves the quality of the canter. This work must be done in the correct frame for it to work so be sure to use side-reins or a chambon.
If you need to get his head low, turn in with one rien, have the other loose, but the one you are pulling in with not tight