Calvin Klein's Ex-Boy Toy Nick Gruber -- Arrested for Coke Possession

Calvin Klein's special male friend Nick Gruber was arrested for cocaine possession in New York this morning following an altercation with another man ... TMZ has learned.

Law enforcement tells us ... the 22-year-old model -- who used to work in gay porn -- was busted at his home at 4:58 AM after someone called 911 to report Nick punched a 20-year-old man in the face.

When cops arrived to the scene, officers say they observed a cut on the victim's face ... and arrested Nick for misdemeanor assault. Cops say Nick resisted by "flailing his arms in the air."

Cops say they noticed Nick suspiciously reaching into the front of his pants ... so they conducted a strip search back at the station ... and discovered cocaine on his person.

Nick is now facing 3 criminal charges -- including assault, resisting arrest and possession of a controlled substance.

We're told Nick is still in police custody at the station.

According to multiple reports, Gruber and Klein "dated" for nearly two years before taking a "break" back in January ... though Klein has not publicly addressed the relationship.

11:59 AM PST -- Moments ago, we shot Nick being escorted out of a NYPD precinct in handcuffs. He was placed into the back of a waiting cop car. He's most likely headed to central booking ... to complete the arrest process.