My two kids-currently-in-braces cheat. Seriously. After he gets over the initial mouth pain, which in this house lasted a full month to get back to full jaw strength without soreness, just pull or cut the jerky into small bites and chew slowly. It is probably not what you wanted to hear, and I usually do not endorse breaking the rules of the ortho, but if you truly make the bites small and chew slowly, letting the moisture and enzymes of the mouth soften the meat, it totally works with jerky.
Otherwise, perhaps lots of breakfast sausage links or patties, bacon strips, or thin-sliced hard sausages if you can get your hands on some good stuff.