Reading your answer about him being hard to get into the canter tells me that the horse is not sensitve to your leg in the first place. He needs what the dressage masters call, "the lesson of the leg". This means you need to reeducate him that leg on means MOVE! NOW!
You will need a crop or whip for this. And if you are not secure, you may have your trainer do it.
Before I go on, I am assuming that your horse is totally capable of cantering on his own and has NO physical issues that would discourage him from cantering, right????
Ok, now get on him. From the get go, when you ask him to walk off, if he doesnt' move into a smooth walk you will lay that crop on his hiney!
You ask once with a gently squeeze of your calves, once again with a kick of the ankle (not too far back) and if that doesn't get a prompt reesponse , lay the crop on. Don't be shy, do it with authority. Your horse will leap forward, so be ready and don't pull him back , just let him go forward as fast as he likes, for a few steps, then ease him back to the gait you asked for.
When he slows down, you ask , tell and demand (with that crop). You want those three steps to be fairly close to each other. Don't let there be a lot of time between them. You want to wake up the horse. It's ok if his reaction is more than you asked for, don't let that scare you. You are used to a dull horse and a spritely horse might feel odd. But keep at it. When he goes forward, you go with him. Let him coast. He is doing the RIGHT thing , so make that easy for him.
If you really commit to this you will get him so ready to move that all you will have to do is "flutter" your ankle against his side and he will pick up the pace. But, you must be serious about it, and consistent.
Once he is more responsive, he will step into the canter more readily.
The half halt is also very helpful to prepare him to canter. But if you don't know how to do it, at least have a horse that has a good "GO" button on him.