Well because of it's design, it applies light pressure to the chin to remind the horse to keep his mouth closed. If he does open it, the pressure increases and at a certain point before he can actually get it open it just stops the horse all together. At the same time it is applying that same pressure to the cheek bones, working as a sort of 'block' from moving his jaw to either side and thus crossing them :P that paired with the correct bit can help prevent him evading the bit in a number of ways. (I find fuller bits to be better for this but I realize that you do have a fairly strong pony so this may not work for you)
The thin mouth sweet iron may very well be a good choice. If you have one with a french link, that will enable to you work seperately on each side of his mouth for better turning control. The traditional doesn't give you as much control, and it really isn't any stronger than the eggbutt minus the fact that it's thinner, working on a smaller area of the mouth- but he may like the feel of it better.