Why would you have him cantering before he is capable? That sounds irresponsible. He needs to learn how to sit the trot before he should be cantering. I am guessing with his position the horse may be going to fast for him
I'm not the head instructor; it's all up to boss lady and she likes to give people more credit than what they're worth. I'm going to try to have a conversation with her about his position and things I've learned here to help fix it. I also want to try a variety of other posture and "soft hand" exercises I know about. According to some of the volunteers (who has owned horses in their own past, I have not), I'm simply ignorant and can therefore not help another rider. Most of my tips (many of them I learned from those very people) are thrown over shoulders.
Example of the too-soft-heart: My boss took in a volunteer who claimed to be a competitive calf roper. His favorite horse (and the one he rode most often) was a Tennessee Walking Horse, smooth as glass. One day he was in the at-the-time-new round pen exercising this horse (who's our lesson horse); another volunteer had an Appaloosa, not as smooth, who was acting up. So my boss had the first boy (the "calf roper") ride this horse. When they began to trot, "Calf Roper" began to flop all over like a dead fish! (He would literally fall backwards and nearly touch his back to the horse's hip.) My boss then discovered: He duped her.
In my personal opinion this kid shouldn't even be trotting right now. I'd like to get him into the arena -- by himself -- and work him out a little more. He has the potential to have a fantastic seat but is, unfortunately for him, distractable -- so the seat he does have is lost in a matter of minutes.
Since he rode once or twice with us before he volunteered and rides with his grandfather (which, to my understanding, they only walk or gallop) I don't know how
many times [with useless or no instruction], my boss took his word for it that he knows how to ride. If I'm starting to see that he's not what he made himself out to be, she should be too. Hopefully the coming discussion will help us sort this situation.