HESPERIA - A 4-month-old Southern California boy remained hospitalized in a coma Wednesday after being beaten allegedly by his mother's boyfriend, authorities said.

Tyron Bookman, 24, of Hesperia, is accused of abusing his girlfriend's infant son so badly he was left with a fractured skull, according to the San Bernardino County Sheriff's Department. He pleaded not guilty Wednesday to a felony child abuse charge.

The boy's mother told NBC4 her son, Zachary, is the "light of her life."

"It really hurts me, that I can't believe that he's in that coma, and thinking that he hasn't moved or (done) anything yet," said Rebekah Rinaldi, who said she is pregnant with Bookman's child and just recently broke up with Bookman.

Authorities first found the injured infant after Bookman allegedly called 911 on April 8 about a 4-month-old boy left in his care who was having trouble breathing, officials said.

When authorities arrived at the Tamarisk Avenue home, they found the boy unable to breathe on his own, officials said. Paramedics rushed the infant to a nearby hospital where doctors discovered he had a fractured skull and showed signs of shaken baby syndrome.

More tests performed on the child showed he had a fracture to the occipital bone, a complex fracture to the parietal bone, a subdural hematoma, and a bruise to the tip of his penis. The infant remained in a coma in critical condition on Wednesday, officials said.

Deputies said Bookman's girlfriend left her infant son and 3-year-old daughter with him while she ran errands. When she left, the infant boy was buckled into a car seat and left in front of the television, officials said.

Detectives said Bookman used an "unknown object" to hit the left side of the boy's skull several times, adding that the child's subdural hematoma was "consistent with sudden acceleration and deceleration."

Rinaldi said Bookman called her from jail Wednesday and blamed the boy's injuries on her 3-year-old daughter.

Bookman is being held in lieu of $500,000 bail. He pleaded not guilty Wednesday and is due back in court April 27. (KNBC)

Story Date: April 18, 2015