Many horses will be tight somewhere so even if it is not a big issue the physio should help him and show you how to work on him if it is a one off.
Re cantering, my boy [aged eight] has needed quite a lot of flat-work to get his canter going, this includes long reining in a circle before riding, and asking for contact. No use on a single lunge rein as he tries to nap when he feels uncomfortable on the right rein.
He needs to be ridden with outside rein contact on the left rein, but tends to lean against the leg on the right rein corner, so you see he really needed an expert [my BHSII] to sort out his problems. Commands must be clear and if he does not do as asked, he has to half halt or checked, and asked again.
We use a schooling whip to back up leg aids, also verbal scolding or encouragement. He gets very little walking in the school as it gives him time to think about naughty things to do!.
He is now offering the canter on the long reining, and is much more forward going at all times, he has always been the type to revert to a nap when out hacking, he can be spooky, and then this develops in to a nap.
A confident rider with an good seat is essential with this type, but there is also the problem of moving [same thing happened to me], a few regular lessons will help, if you are struggling, ask your instructor to ride him for the last thirty minutes of the lesson, while you ride him for the first thirty minutes.