The Jonas Brothers star admits he was startled by Nick's skeletal frame and knew something was wrong with his brother, who was quickly diagnosed with diabetes.
"One day, me, Joe and Nick were all sharing a room and Nick is changing," Kevin recalls in the new Jonas Brothers documentary "Chasing Happiness". "I look at him and I could see every single bone in his body."
Nick's parents took him to see a doctor, who urged them to go straight to the hospital.
"I passed out in the hospital room because I couldn't see him, like, all hooked up to all these wires," Joe Jonas recalls. "It was, like, a really life-shifting moment for everybody."
The brothers' mother, Denise, reveals Nick changed after his diagnosis and suddenly became a much more serious person: "Nick, as we had known him - happy, smiley, smiling, singing at the top of his lungs when he wakes up, he was different after that."
But he didn't let the health scare derail his pop dreams - the brothers went straight from the hospital to a gig, opting to keep Nick's diabetes diagnosis to themselves for a while due to fears of being dropped by their record label bosses.