Lunging is definitely important. If he is an ex racer like my horse, he will need to get used to balancing in a whole new way. He now has to carry a rider and its definitely different for him. I lunged my standardbred a LOT. ITs important.
Also for cantering, reward every single stride. Some pick it up easier, some don't. My horse raced A LOT and won like 800K so I know it was difficult for him to break out of his racer ways =P
Make sure you're experienced enough to handle it, some horses will be good tempered, but some will not like the scary feeling of being totally unbalanced at a canter until they get used to it and gain balance. So be prepared. Also be prepared for some serious speed LOL
Definitely start with a lot of circles. Standardbreds that are off the track are very stiff and won't bend readily because they need to stay straight most of the time. So do a lot of circles to reeeally get them flexible before you ask them to canter. Patience is really reallly important
OKay so when you're ready to canter be ready for tripping and its only going to be a few strides at first. But be very rewarding. When they were trained a lot of riders would snap the reigns when the horse cantered at a young age. So give him his head. If you pull back to balance yourself or slow him toooo much he'll consider it a reprimand and stop immediately , he'll already be nervous in the gait.
Let him build up the necessary muscles on the longe. He'll need to get used to this. So a lot of longe work in the canter will definitely be helpful
Collection- don't let him trot fast into a canter, keep the trot REALLY slow and ask in a corner. I can't emphasize the corner thing enough, they wont wanna pick it up on the straight at first. Of course they will soon enough, but begin in corners.
Aids - you need to use the exact same aids to ask for his canter every single time. Every rider is different, some emphasize different things more than others, which is fine, but you're horse needs to be fine tuned TO YOU. So be sure to stay very very consistent.
IF you have anymore questions contact me anytime =D I've successfully retrained a standardbred and am still training with him, so I can easily answer any of your questions and I certainly wish you luck