So glad you were able to canter a bit! When your saddle situations works out, you could try asking for the canter from the two point position with a steady contact on both reins (you can even "bridge" the reins on his neck). This will most closely mimic how he was cantered during his time on the track. When you want to come back to a trot, relax your contact on the reins, stretch your body tall and settle back down into the saddle.