John C. McGinley is sitting in the office of his Malibu home when his son Max, 4, appears in the doorway. "You come here!" McGinley coaxes. After Max approaches and places his arms around his father's neck, McGinley begins singing "The Ugly Duckling"--"Say! Who's an ugly duckling? Not I!"--as Max quacks on cue.

Hang on: Could this be the same guy who makes interns quake as Dr. Perry Cox on NBC's hit medical sitcom Scrubs? And who, in some 50-odd films (including six with director Oliver Stone) and TV movies, has played not-so-nice guys ranging from a battle-hardened sergeant in 1986's Platoon to a smug broker in 1987's Wall Street to a rabid serial killer in 1997's Intensity (based on the Dean Koontz novel)? Yes, but no more murderers, please. "Not after Max," McGinley says. "I'd rather play Dr. Cox, who every once in a while [reveals] a crack in the armor and you see some compassion." His Scrubs cohorts see that and much more in McGinley. "John has a lot of alpha-male qualities," says Zach Braff (first-year intern John "J.D." Dorian), "but with his son he's like this big teddy bear."

Mixing the tough and the tender is a skill the 42-year-old actor has honed in his years of caring for Max, who was born with Down syndrome, a chromosomal abnormality that has left Max with some developmental disabilities. McGinley, who has joint custody of Max with ex-wife Lauren Lambert, 36, a law student, lives for the weekends he shares with his son. "Everything for me," he says, "is about Max."

That's evident from a tour of McGinley's four-bedroom beachside house, which has been "Max-proofed," he says, to protect Max from hurting himself when he falls. (Poor muscle tone is a side effect for some Down syndrome patients.) There's rubber flooring in Max's playroom, and his bedroom mattress rests on the floor. Although he can say only a handful of words (including "bye," "purple," "mom" and "dad"), Max attends a mainstream preschool and a therapeutic gym class to build strength.

McGinley himself is no stranger to learning disabilities, having been born with a mild form of dyslexia. But the Millburn, N.J., native--who is the second oldest of five children--still performed well in school and was always tapped to emcee the family talent shows enthusiastically applauded by his parents, Gerald, 72, a stockbroker, and Patricia, 67, a retired schoolteacher. "As soon as I could talk," McGinley says, "I wanted to be heard."

During a summer at sports camp, McGinley starred in a play, "and that's when the light went on," his father says. After graduating in 1984 from the theater program at New York University (where future ER star Eriq La Salle was a classmate), McGinley worked Off-Broadway before landing the role of Sgt. Red O'Neill in Stone's Platoon. He went on to specialize in supporting parts: "the next-door neighbor, the co-worker, the guy who gets killed to propel the hero into action," he says.

It was in 1996 on the set of Nothing to Lose that McGinley met Lambert, then a production assistant. The couple married in February 1997, six months before Max was born with a single crease on each of his palms, a telltale sign of Down syndrome. A blood test confirmed the diagnosis. "We cried until we had no more tears left," says McGinley. "Then you see this kid looking up at you, as if to say, 'If you guys are done now, I could use some help.'"

Both weathered Max's health problems, including sleep apnea and a seizure disorder brought under control by medication. Though McGinley and Lambert divorced in December, Max's condition wasn't a factor, McGinley says. "It felt more like we were great parents and not good partners."

McGinley says he isn't involved with anyone right now: "I'm with Max." Tucking his son in at night, "I tell him stories and sing to him," he says. "When he brings his arms around your neck and you can feel those legs wrap around you, that's as good as it gets."

--Galina Espinoza --Lyndon Stambler in Malibu