I agree with the people who said to mix it up in the ring. Lots of transitions in the walk and trot are your best bet to get his focus.
As far as earplugs, they are not allowed in competition. It’s not the greatest idea to practice so differently from how you will compete. In addition, many horses strongly dislike having things shoved down their ears. I tried earplugs with my first horse who used to love having her ears touched and brushed. After a few days she would rear if I tried to touch them. If you really feel the need to do something I would say a sound proof ear bonnet is a much safer bet.
As far as bitting, use a loose copper bit. The point of a loose ring is that it’s rotation encourages salivation, which loosens the jaw and relaxes the mouth. Copper bits do the same as well and they are legal in dressage. A Dr. Bristol bit is legal and is somewhat strong or a French link which is often used on young horses because the center piece gets their attention. Bits with rollers have also been known to work on horses who’s minds seem to be flighty because the roller gives them something to play with.
As far as your cantering problems, I had the same exact problem with my mare. We would stick to lots of transitions in the canter. We would only canter for 2-3 strides at a time. When she began to get excited about canter transitions, I would move her down to trot and walk transitions so she didn’t know what to expect. I would also do a fair amount of lateral work such as leg yields or shoulder-in to keep her attention. The only thing about lateral work is that you don’t overdo it and anger the horse.