baby hitting his head in crib?

Angelica • June 2016👶🏻💙
My son is 6 months old and he does this thing here he scoots up in his crib while sleeping or awake and hits his head on the inside of the crib like where his head is facing (like basically the crib headboard). How do I stop him from doing this or what can I get so he doesn't hit his head so hard?!