first of all, pretty much all horses will become more excited at the canter, especially in a group. It doesn't matter if it looks like the side of a road, though the similarity to past places where your horse was raced is probably a part of the problem . But, the canter is always a place where horses tend to flip from the more thiniking side of the brain to the more reactive side of the brain.
That's why you have to have problems worked out really well at lower gaits, so that when you go into the canter, they will remember the training better, since it's more automatic.
Since your horse has been trained to come behind the bit you will probably have to work on slowing him without using a straight pull back on the reins, since he knows how to avoid that. Are you using a snaffle? (and I dont' mean a shanked snaffle). If so, utilitze lots of small circles to slow him when he starts to lose him mind at the canter. So, if he wont slow to the rein, then circle him until he does, then let him come out forward (not riding the reins) and if he speeds up again, circle again. Don't ride the "brakes", but utilize him working harder to circle to encourage him to maybe want to stay slow of his own accord.