Calling All Mommies!

I need some help/advice. My one-year old son moves a lot while he is sleeping. I hear him constantly whacking his head on the bars of the crib. Usually it doesn't wake him up, but sometimes it does and he cries. Also, some mornings he has indentations on his head where he slept against the bars. It doesn't seem to bother him, but it looks really uncomfortable. He does not have any bumper pads because he already tries to climb out of the crib as it is, and I don't want to give him a "leg up" so to speak. Any ideas??