T.b retraining question ?

I'd do alot of trotting untill he's relaxed as... and then when you ask for the canter start rocking in your saddle sitting deep. He needs that contact with you in the saddle and just time. I'd give him a few weeks to settle and pretty much detox :) They can have some crazy things being fed and injected into them and its best to give them a chance to come down so to speak. I wouldnt be cantering him for a few weeks. Just lots of walking and trotting till he's relaxed and calm. Have him walking along and get a horse and rider to trot past him...stuff like that to teach him he doesnt always have to be in front. With a racehorse they are usually on a tight rein and when you shorten your reins it means they are about to gallop...keep all that in mind. When you do canter have him in a small yard, somewhere with not really enough room to gallop but enough to canter in big circles... have fun!