the way to work on a canter like this, that is unbalanced, is to work on the trot.
you go back to the trot and get the horse able to trot circles, serpentines and spiral in and out, with a good bend (not overbent), even trot tempo and connected to the bit with his jaw flexed lightly to the inside, soft on the bit.
Do lots of trot to walk to trot to walk to halt and on and on. Do them on circles. TRot fast, trot slow but have him mostly trotting on a curve of some kind and always have him correctly bent and pushing from behind. Not a real headset or collection, but not slogging along on draping lines either.
Wehn you feel he can trot both directions almost equally well (no horse does them exactly equally well except those super well schooled) then ask for the canter, in the corner, and canter just a bit , then back to trot. Only canter as long as the hrose is balanced. Cantering on and on with the hrose poorly balanced , or counter bent will not improve. is so, go back to trot.
end on a good note.