Try the "conveyor belt" game. Start out with one horse in the lead and the others in line behind him. Ride out at a walk or trot for about 100 yards and then the first horse peels off and goes to the rear. Ride on with the 2nd horse in front for about 100 yards and the peel off and go to the rear. Keep repeating this until everyone is calm and accepting, might take more than one trail ride to accomplish but eventually every one will be able to ride in any position without the horse being anxious. Then when you want to canter and he gets too fast, take him to the rear and make him slow down, if you've done the work, he shouldn't get anxious and want to crowd.
Since he's not your horse, of course, this can all get undone if the owner lets him ride up someone's tail. Nature of the beast when it's not your horse.