I would put a regular snaffle in his mouth. I personally hate Kimberwicks. In my opinion, if you can't ride in a simple snaffle, then something is very wrong. If he gets impulsive put him on a circle. The circle's consistancy will calm him down, and when he relaxes stop and let him rest.
I agree with Sara that you need to open your hips up more. Think about your legs as being wet towels and just kind of draping around his sides. The one thing I disagree on is riding more on your seat bones. In the canter, the horse rocks 70% of his weight onto his hind quarters. If you sit on your seat bones, your weight is more forward on the horse's front end, causing it to become more difficult for him to rock back. You need to sit more on your butt and allow your legs to be nice and loose, but not swinging. Relax your shoulders and your back, especially your lower back.