Brent Troy Bartel called 911 Wednesday, telling the operator he "shed some innocent blood."
When asked why, he said because "it's a holy day."
Officers responded to the house in Richland Hills, Texas, after the boy's mother also called 911 from a neighbor's house, saying her husband was hurting the child, KXAS-TV, Dallas/Fort Worth, reported Thursday.
Police said they found the boy wearing only pajama pants and with a pentagram covering most of his back carved into his skin.
The boy was treated at Cook Children's Hospital and police said the carving wasn't deep enough to require stitches.
Police arrested and charged Bartel with aggravated assault with a deadly weapon on a family member. He was being held on $500,000 bond, KXAS reported.
It is not clear what Bartel meant by calling Wednesday a "holy day." He could have been referring to the repetitive date, 12-12-12, the fact that Dec. 12 is the Feast of Our Lady of Guadalupe, or that some doomsday prophecies have drawn attention to Dec. 12, 2012, and Dec. 21, 2012, as significant dates, KXAS said.