The Bronx couple slain along with their 5-year-old boy were on a “paint and sip” date night hours before they were butchered — and cops are eyeing the man’s teen son as a person of interest in the grisly triple stabbing, law enforcement sources said Monday.

The sources said the 19-year-old son of victim Jonathan Rivera checked into the Westchester County Medical Center in Valhalla following the murders.

The upstate college student has not been named by authorities and has not been charged in the slayings that left friends and relatives grappling for answer Monday.

“What happened to them doesn’t make any sense,” family friend Eusebio Baez said outside the Mott Haven apartment building where the bodies of Rivera, 38, his girlfriend Hanoi Peralta, 33, and the couple’s young son, Kayden Rivera, were discovered early Sunday.

Less than 12 hours earlier, the couple had gone on a date as they tried to mend their relationship, said Baez, who has known Jonathan Rivera for 20 years.

“The night before, I spoke to him. He was at a paint and sip with his girlfriend … They posted on social media having a good time,” Baez, 33, told reporters.

“He was showing everyone, ‘Look how much better her painting is than mine.’ They were both happy.”

Rivera was a doting father who spoiled little Kayden and devoted all his time to reuniting with the boy’s mom before their tragic deaths, Baez said.

The last message he received from his old friend came at 7 p.m. Saturday.

At around 6:40 a.m. Sunday, police responding to a 911 call found Rivera dead in the first-floor hallway of Peralta’s building at 674 East 136th Street, with multiple stab wounds throughout his body.

As they investigated, the officers looked into the window of the nearby apartment unit — and broke in after spotting Kayden, sources have said.

The boy was found on an air mattress inside, stabbed so viciously he appeared disemboweled, according to sources.

His mother was found in a bedroom, also with multiple stab wounds, cops said.

Baez said Kayden was the youngest of Rivera’s four children, with two older daughters who live in Connecticut and the older son — now suspected in the slayings — who is attending Oswego College upstate on a scholarship.

Rivera worked two jobs — including as a janitor at his son’s Bronx school — to support the boy and Peralta and spent all his time and money to ensure both were happy, Baez said.

“Jonathan was never in trouble,” he said. “He barely had time. His free time was spent on his kids and snowboarding. He’s not even a guy who goes to bars or anything. He had his first IPA last year.

“The dude, I’m telling you, was a solid man.”

The couple split about a year ago but had begun dating again in recent months to try to patch up their relationship, he added.

Baez said Rivera had been living with his father elsewhere in the Bronx but would spend more and more time at Peralta’s apartment.

“She lived here but he’d come over to babysit the kid, clean the house and make sure everything was good when she got home from work,” he said. “He used to bring her out when we had parties and little get-togethers. She was a sweetheart.

“He loved that woman so much. He worked his ass off just for his kids and spent any little money he had left over. He would spoil her,” Baez said.

Rivera was “a sweet guy with a golden heart” who would spoil Kayden, Baez said, noting “40% of that living room was catered to his son’s toys.”

He remembered the slain youngster as “energetic” and “a little sassy.”

“He loved chicken nuggets,” Baez said. “He loved Happy Meals at McDonald’s. Just a regular 5-year-old who definitely didn’t deserve what happened.”

Rivera “would spend so much money on toys for his kid,” Baez recalled. “Any toy he wanted — Sonic, Ironman. I’m getting goosebumps. He cared for that kid so much.”

Additional reporting by Amanda Woods