Calvin Klein’s younger boyfriend Nick Gruber was arrested after punching a man, also named Calvin, he took home after a party.

Gruber, the 22-year-old ex-porn star paramour of the 69-year-old fashion icon allegedly hit Calvin Swint, 20 — who’s believed to work as an intern for New Jersey Sen. Frank Lautenberg — on the balcony of his Greenwich Street penthouse during a 4:20 a.m. fight yesterday.

Sources tell The Post’s Kirstan Conley that when cops arrived, Gruber flailed his arms to avoid being cuffed and shoved his hands into his pants. Police found cocaine in Gruber’s underwear when he was later searched.

Other sources tell Page Six that Gruber — who had an on/off relationship with Klein and was recently reconciled with him — has some explaining to do since “Calvin paid his rent two years upfront, and was just saying how happy he was in the relationship.” Klein is also sober.

Gruber was arraigned last night at Manhattan Criminal Court on a felony charge of possessing a stimulant and two misdemeanors — resisting arrest and assault with intent to cause physical injury.

But Swint told The Post he didn’t want to press charges: “I don’t want to be involved in any of this. We’re good friends, end of story.”

Swint added, “Everybody expects Nick to be somebody. He’s a great guy, he’s a funny guy — he’ll make it, he was talking about going to college. I’m absolutely not in a relationship with him. He’s like my brother, so if he was angry, I could understand why he could take it out on me. We all get depressed sometimes. I feel for him. No one else was on the balcony, just us. It happened, and then I left. I felt embarrassed . . . I’ve never been in a fight before. I went downstairs, and the doorman asked if they should call the police because my eye was bleeding . . . I’m sorry this happened.”

Klein’s rep couldn’t be reached. Gruber’s lawyer, Ben Brafman said he was “confident” the charges “will be dismissed.”