First, good job so far with this horse, you obviously are doing something well considering where he came from and the results you're getting already.
But, take your time. As mentioned before, re-training a horse takes longer. I absolutely wouldn't go to a more severe bit. Maybe try different types, I know my arabians have a narrow mouth and prefer a double-jointed bit. IMO, bits are not breaks but communication tools. This horse didn't learn how to stop. I'd be very persistant in doing down transitions with your seat, then your hands (no pulling) and I'm sure the horse will get it, they are smart... Practice an emergency on rein stop, just in case.
For leg yield, one side stiffness and bolting at the canter, I would do a lot of ground work. They must yield on the ground first. On the lunge, you work the difficult side a little more than the easy side. Probably won't become perfect at his age, but you can make it better.