Baby hitting head (hard) off crib.

Jessica • Mom of 2 boys, 1 girl🧒🏼👦🏼👦🏼26 years young. Engaged💝💍
My son is now strong enough to kick his legs and throw himself backwards while laying on his back. This now causes him to throw himself head first into the crib sides then he starts screaming. I know I can't use bumper pads. What else can I do to prevent him from causing any damage to his head?