I use them on all my snaffles for that reason too. Happens sometimes when working high-speed or working cattle............. or in a disagreement with a young horse ;p LOL. The way that I always adjust mine is to put it on and then pull the rings out to the sides. I like for it to be just a little bit loose (about 1/2 inch to a whole inch longer than the mouthpiece itself). It really depends on how the horse's mouth and head is shaped. It will need to be longer for a horse with a bigger chin and shorter for a horse with a smaller chin.
If I may make a suggestion. You may want to take him back to the roundpen for a while and work on his suppleness to the bit. He should be willing to bend his nose to your toe with little or no resistance. Putting a curb strap on the bit will definitely help in the beginning.