Lunging helps them to independently balance themselves without relying on you to help them so try lunging! You could also try picking up your canter over poles!
Work on his balance, if he tries to trot faster, stop him and then try again ☺️
He might be unbalanced so do some trotting poles and cavaletti and then on a flat do a lot of transitions as I mentioned yesterday. Don't over bend him, let him have quite loose rein bcos horse is using its head and neck to keep balance. As he will be getting more balanced you can start licking your reins up more and more
He’s a very forward horse, but he takes many strides to pick up the canter. I know he’s able to pick it up quickly because he does on the lunge line.
Work on keeping him forward in trot. Do a lot of transitions and direct transitions to make him more sensitive to your leg. Do the same with canter. If he won't go the way ypu want him to bring him back to trot and ask again and again and again. Use a whip and voice to encourage him more. By the end of one session you should be able to see difference
What exactly is the problem? Is he taking a few strides to get there? Is he going into canter but just lazy?