If he's not balanced he won't be able to slow his canter either, because he will be on the forehand. My horse wasn't off the track, she is a Paint I got from a rescue, but she was the same way...I have been working with a trainer now for a year and we're just now getting her canter slowed down (first we had to slow her trot haha!!) She had to gain muscle and be able to carry herself on her haunches which she couldn't do at first...and now I'm just trying to teach her to rock back and do that at the canter.
I'd suggest a trainer if you have access to one, and if not, of course you're doing the right thing seeking advice on here first of all :) Try lots of transitions from trot to canter, canter to trot, trot to walk, etc. and get him working from behind, assuming he is not already. It will take time, but if that's the issue it will get better I promise :)
It may be, as was mentioned, that because he was taught to haul *ahem* BUTT...he may just want to run and that's all he knows! A trainer would be able to help you retrain him in that case, or maybe someone else on here with green OTTB experience!
"The times when you have seen only one set of footprints in the sand, is when I carried you..."