Unfortunately I am in work so can't view the youtube video right now- I will check it when I get home.
One thing I would like to say is trotting poles will NOT encourage a horse to drop his head, and a horse that is lacking balance and confidence should not be asked to go over trotting poles.
Groundwork such as lunging is hugely beneficial to horses that have green or unsteady canters.
My old mare was 6 when I bought her, skin and bone and 18.1hh and a rectangle shape. Building her up wsa a nightmare, she had no balance. But we got there.
Lunging with gummi reins that run behind the ears, through the bit and attach to the lunge girth help a horse to accept the contact and lower the head, without forcing them in to position. From the ground you can drive them forwards.
Vocals are a big key to lunging, and your horse should learn 'walk, trot, canter, halt, forwards, slow.' If you aren't very knowledgeable about lunging, I would advise getting some lessons.
Over time and depending on how the horse accepts the gummis and contact you can move on to side reins, and then a pessoa device system which encourages the horse to work from behind.
Before attaching any devices, warm your horse up for 10-15 mins in w/t.
Cantering on a green horse should always be on the middle circle. Ask for canter at E/B so they have the chance to use that bend and the school wall to balance themselves and go on the correct lead. You don't want to sit 'heavy' in the saddle, but sit with a light-ish seat so you keep the contact and drive, but you don't over work your body. That way, when you want your horse to trot you can sit heavier in the saddle so they learn to use the transitions off your body.
Lots of walking will also help build him up, and free jumping once he is more confident within his gaits.
Going to try and load the video on my phone so I can have a quick peek...