If he canters off, teach him to one rein stop and shut him down the second he starts to take off with you. It starts on the ground in a halter teaching him to bend lightly. Progress to bit from the ground. Then practice practice practice at all 3 gaits in the arena. This becomes a great tool...
I'm working with a 4 year old right now and we just overcame some pretty major spooking issues. It started on the ground doing lots of desensitizing. We started with just desensitizing to flinging a rope all over her body, around her legs etc, then desensitized to long lines (hard for a lot of...
I have been lightly riding my 4 year old. She has somewhere between 30 and 40 rides on her now. Done tons of natural horsemanship groundwork and desensitizing before I started her. We had 6 feet of snow all winter where I live but that didn't stop me from doing desensitizing work with her...