A Brooklyn teen has been arrested for assault in connection with the death of his girlfriend’s 2-year-old son in Cypress Hills, cops said.

Tyree Wright, 19, was taking care of little Jacqil Degraffenreid at his home when the boy’s mother, Natalie Garner, 21, left for her catering job at about 9 p.m. Friday, police sources said.

When Garner returned to Wright’s Shepherd Avenue home at about 2 a.m. yesterday, she said, her son was acting strangely, police sources said. When the boy then slipped into unconsciousness, she called 911, the sources said.

The boy was rushed to Kings County Hospital at 4:34 a.m., where he was pronounced dead.

Wright was charged yesterday with assault and acting in a manner injurious to a child. That was before an autopsy was performed on the boy.

An autopsy has since shown that Degraffenreid died from lacerations to his liver and pancreas and internal hemorrhaging caused by blunt-impact injuries, according to a spokeswoman for the city Medical Examiner’s Office.