An Upper East Side dad said his son was bitten by a large pooch owned by a Chapin School parent outside the elite all-girls institution — and claims officials there withheld the identity of the dog’s owner for weeks.

Dr. Kent Dunn was walking his son, Elias, to a swim lesson around 4 p.m. on March 16, when a 53-pound Portuguese Water Dog named Boris suddenly charged at the 6-year-old outside the $60,000 a year, East 84th Street school.

The dog “was the same height as Elias’ head. Thankfully it didn’t go for his face,” recalled Dunn, 47, who works as a medical director at Pfizer. 

“The dog bit me and my leg hurts,” Elias said at the time, according to his dad. “My leg really, really hurts, I can’t walk.”

The dog’s owner, a dad waiting to pick up his daughter from the school — which counts actress Sigourney Weaver, designer Vera Wang and Jackie Kennedy among its alumni — was unsympathetic, Dunn alleged.

“He looks at me dismissively, he says to me, ‘I already said I’m sorry. What do you want me to do about it?’ ” said Dunn.

A witness who saw the leashed dog “lunge” at the boy said the owner, “had an attitude. At least apologize to the dad with his son.”

A woman, believed to be a school administrator, “stormed out” of the building and confronted Dunn, bizarrely ordering him to leave and threatening to call security — while the dog owner left without giving his name or information, the dad alleged.

Elias had a small bite on his hip, according to a police report.

He didn’t require stitches.

The child was taken to the emergency room and had to endure tetanus and rabies shots before the family hired lawyer Mark Seitelman, who asked Chapin for information about the dog owner.

That’s when the school handed over the dog’s vaccination record, with the owner’s name redacted.

“Their lack of appreciation for the seriousness of this, and their lack of appreciation for our ability to do something, is very clear — we’re demanding Chapin do the right thing,” Dunn said.

Elias’ mom, Ji Won Kong, 47, said Chapin’s stated values of “bravery, compassion, service and respect for others” are bunk.

“They’re protecting this guy, they’re just letting him get away,” said Kong. “It’s a load of hypocrisy.”

After outreach from The Post, a Chapin lawyer identified the dog owner, Seitelman said.

The dog owner didn’t return messages seeking comment.

“We’re preparing a lawsuit,” Seitelman said.

The Chapin administrator did not respond to a message seeking comment.

A Chapin spokeswoman later denied her the administrator’s involvement, and declined further comment.

Additional reporting by Joaquin Contreras and Georgia Worrell