There are many ways to skin a cat, but this sounds like a balance/confidence issue. I would get up off his back (2 point) and just allow him to canter under you. A big circle or laps around a full size arena is ideal. Keep a steady contact, and every few strides give him a little check with the rein and hold through your back.
This gives him a chance to find his own balance in canter, and gradually he will become more comfortable to come back and begin to carry through his hind legs.
This is the first thing I would do, before starting smaller figures and transitions which will improve the quality of the canter and fine tune the balance. Allowing the horse to find its own balance has always worked well for me, once they are confidently staying in canter and allowing me to check their tempo and rebalance them lightly, I will start coming back onto a 20m circle and play with my favourite canter improving exercises - rapid trot-canter-trot transitions, and transitions within the canter.
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