You can use a Pelham, with a curb chain that adjusted correctly, links lie flat and hang loose below the chin groove, coming into action against the jaw only when the shanks have rotated due to rein pressure. The point at which the curb chain engages varies with the individual needs of the horse, but contact at 45 degrees of shank rotation is a common default adjustment.
Learning how to half halt will encourage the horse to properly lower their head, and lift their back.
Are draw reins fixed or do tgey have some give in them?
Hi! I use draw reins as a riding aid to help encourage them to use their backs properly, this in turn will keep their heads down, I also use a bungee when I lunge him to get him used to working in that outline! Hope this helps!