I agree with PaintHorse about the use of a tie down. In this case, a tie down can lead to a rearing horse since he can't get relief from your son's heavy hands. A mechanical hackamore, in heavy hands, can break a horse's nose or at least damage it.
If you need him to stay with that horse, for a very mild bit, I would choose a Billy Allen mouth piece in a snaffle. It is very forgiving and can't collapse to form a nut cracker the way a 2 piece can. Billy Allen Bits
scroll down to the snaffle - most of these are leverage bits. I've been using that mouth piece on dozens of horses for the past 15 years and never had a problem with it.
For training, I've seen the use of light string or yarn attached to the bit and the reins attached to the yarn. If he pulls too hard, the yarn breaks. It's a training exercise that is first done without a horse then the student is moved up to a horse in an arena. I certainly wouldn't do it on a trail ride or in a pasture where you have no way to control your horse.