As we all know, Johnny is a bit off. He has been since around his first appearance way back in chapter 1, where Max lands on Johnny while going to school.
When Max hit Johnny in the head with his scooter, he's noticeably injured. Stephen even comments on it on the next page, where Johnny seems perfectly fine. This is likely because of the spirit that has attached itself to his head, who we later learn is a Doctopus, which can relieve pain.
However, Doctopi don't actually heal anything.