If you could avoid the tie down it would be best, in my opinion. Tie downs build bad habits and bad muscles in horses. How about just a snaffle and nothing else? That way, if your son is heavy handed, the horse wont get the pull on the rein magnified, as it does with any shanked bit (traditional western curb). A snaffle give only a one to one ratio of pull on the reins to pressure felt on the bit. And work with your son on learning how to apply pressure slowly and wathc the horse for the response, he might be able to release and reward her a lot sooner than he thinks, and take up the pressure a lot slower, too.