Lunging, lunging and more lunging is going to be the best way to fix that. Gradually introduce side reins to encourage an inside bend and when he is consistently getting his leads on the lunge, only then I would start to ride him in the canter. Also riding him in just the walk and trot and working on the suppleness, rhythm, relaxation, contact and impulsion of the horse is going to immensely help his canter. Do a million transitions, literally. If you've gone 20 steps without making a transition then you need to do one, either shorten the stride, lengthen the stride, go to trot walk or halt or transition into a different movement (circle, serpentine, etc..). It will be a long process, but if you do it correctly you'll end up with a great lower level dressage horse, I find most western horses are really well started and responsive.
Good luck! (and get a vet check!)