Gage Bellitto didn’t go home for Christmas. He didn’t call his parents, even though the holiday was also his birthday.

“He was always doing this, ignoring us, so we thought he was making a point,” said his mother, Kyle Bellitto. “We figured he was angry at us.”

Two days later — Dec. 27 — Kyle stopped by her son’s dorm at Columbia University. Police officers were gathered in front of his first-floor room. Don’t go in, they said. A detective broke the horrible news.

“I just couldn’t believe he was gone,” she said.

“I still don’t believe it.”

Police estimate her son died on Dec. 22, five days before his body was found. Gage, who would have turned 20 on Dec. 25, died alone of what authorities suspect was an opioid overdose.

Gage came from a good family and was a smart student who went to the best schools. Yet like 67,000 other Americans killed yearly by opioids, Gage could not escape the epidemic that has spread to college campuses around the country, including the Ivy League.

Jonathan Winnefeld, the 19-year-old son of a Navy admiral, died in a University of Denver dorm Sept. 7 after overdosing on fentanyl and heroin. The next day, Eric Chase Bolling — the 19-year-old son of former Fox News personality Eric Bolling — died from a drug mix that also included fentanyl while attending the University of Colorado.

About 7 percent of US college students have misused non-medical prescription opioids in their lifetimes, and 4 percent reported misusing the drugs in the previous year, according to a national survey by the University of Michigan.

‘He got worse and worse and his health completely deteriorated’

Data about opioid abuse at top colleges like Columbia are scarce. But students at competitive schools are at a higher risk of using opioids to cope with academic and social pressures, said Sean Esteban McCabe, a University of Michigan professor and expert on student addiction.

Gage Bellitto was nothing if not driven.

His parents — both with Harvard degrees — reared him in leafy Bronxville, a Westchester suburb where the median household income is $200,000 a year. Kyle Bellitto is a lawyer. Gage’s dad, Glenn Bellitto, 59, is a finance professional and a member of the Eastchester town council.

From age 14, Gage wanted to follow in his parents’ footsteps. “He had this list of the best colleges in the US and his goal was to get as high on the list as possible,” said Kyle, 57. “When he really set his mind to something, he’d stop at nothing to achieve it.”

Gage also liked to push boundaries. By his junior year of high school, he was regularly experimenting with drugs and alcohol. He threw big parties when his parents left town, left school during lunch to smoke weed in his basement with pals and got into fights, said his friend Will Rabsey.

“Cocaine was really big for him,” said Rabsey, 19, who went to all 12 years of elementary, middle and high school with Gage. “I remember a few times he’d put a line out and I’d say, ‘Damn, Gage, that’s a lot.’ He’d say, ‘No, no I’ve done this before. I can handle it.’ ”

After high school, Gage attended Bates College in Maine. Bates is highly ranked, but it wasn’t good enough for Gage, and he transferred to Columbia.

“You see, dad? Now all of us in the family went to the Ivy League!” Gage chirped.

But his dad sensed something was wrong. When he picked up Gage at Bates at the end of the school year in May 2017, he noticed his son had wrapped his dorm room smoke detector in plastic.

“I said, ‘You’re crazy, what are you doing, smoking pot in here?’ ” said Glenn. “That’s how the summer kicked off.”

As the weeks went by, Gage’s parents saw a big shift in his behavior. They found dozens of beer cans in their basement. Gage openly smoked weed inside and his temper worsened.

“Whenever I’d try to talk to him about drugs, he’d get really upset and wouldn’t talk to me for days,” Glenn recalled. He often told his parents to “f–k off” and slammed his bedroom door.

Rabsey thinks Gage felt untouchable after getting into Columbia. “He told me he was so happy he got in that he was just going kind of buck wild last summer . . . He was often taking something,” he said.

Gage lied to his psychiatrist to up his prescription of Klonopin, an anti-anxiety pill. He’d also been prescribed Vyvanse, a stimulant, and Methylphenidate — commonly known as Ritalin — to treat attention deficit disorder.

On Aug. 28, Gage moved into his dorm at the Carlton Arms on Riverside Drive. He planned to study economics. At first, he seemed normal to his 12 suite-mates. But drugs quickly consumed his life.

“He got worse and worse and his health completely deteriorated,” said Miguel Moya, who lived with Gage. He lost weight, and seemed to live on ice cream sandwiches and Coca-Cola.

On Oct. 12, Gage collapsed in the dorm’s common room and was rushed to St. Luke’s Hospital. Doctors told his parents he’d taken a “dangerous mixture of drugs.” But Gage wouldn’t sign release papers allowing them to see what drugs were in his system.

“That’s when I took my blinders off and thought, ‘This is a big problem,’ ” said Kyle.

But Gage refused to attend an inpatient program.

“We knew he was prescribed things and drank beer and smoked,” said Kyle. They didn’t know about the opioids.

Cops believe Gage got his opioids — which could have ranged from painkillers to heroin — illegally. Where he bought them is under investigation.

However, Gage was still finishing his finals at Columbia, earning Bs and Cs. Kyle planned to pick up her son from school on Dec. 21, but he never showed up.

On Dec. 27, the Bellittos reported Gage as a missing person to Columbia security. “It didn’t even cross my mind that he was dead or even injured,” said Kyle.

“It was way worse than we had ever imagined,” Glenn added. “It often isn’t what you see that’s deadly. It’s what you don’t see.”