A St. Paul man beat his girlfriend’s 4-year-old son while she was at work, leaving the boy with black eyes and missing teeth, according to charges filed Wednesday.

Prosecutors say he tried to pass the injuries off as a bicycling injury — until the boy told hospital staffers otherwise.

Joshua P. Payne, 35, was charged in Ramsey County with malicious punishment of a child, a felony. He’s jailed with bail set at $50,000.

According to court documents filed by prosecutors, the boy’s mother and grandmother took him to the emergency room at Children’s Hospital on Monday night with “significant” bruising on his face.

The mother said Payne, her live-in boyfriend of six months, told her the child fell off his bike without a helmet and hit his head.

An examination turned up more bruises on his arms, back, abdomen and rear end. Some were “consistent with adult finger-tip pressure,” the charges said; others looked to have been caused by being whipped by a cord.

The boy’s eyes were black and swollen nearly shut, the charges said. He also had internal swelling on his head and torso, and signs of healing fractures in two ribs.

The child told hospital staff that Payne had hit him and knocked out his four front teeth, according to the criminal complaint. He said he threw up after the beating. He also told police that Payne had used a spatula at one point.

Investigators found that Payne had been watching over the boy during the day since March while the mother worked. Payne declined an interview with police when arrested, saying “it’s just all B.S.,” the complaint said.

Payne has prior convictions in Minnesota for domestic assault, burglary and drunken driving, according to court records.

Marino Eccher can be reached at 651-228-5421. Follow him at twitter.com/marinoeccher.