Nick Santillo was born with half a heart, but he puts all of it into his love of John Cena, the 15-time World Wrestling Entertainment champ who has been the face of the brand for more than a decade. “He’s my hero,” says Nick, who lives in Runnemede, NJ.

The 10-year-old — whose condition makes it difficult to breathe and has resulted in so many surgeries his father gave him the wrestling nickname “Nick the Stitch” — is one of the hundreds of kids a year who request to meet Cena through the Make-A-Wish Foundation.

The wrestler holds the all-time record for the most wishes granted through the organization, with more than 460. Besides visiting hospitals, he also takes kids backstage at matches and treats them to WWE T-shirts, wristbands and other merch. Nick got invited to a live wrestling match, where Cena brought him into the ring and announced his name to the crowd.

But a towel — featuring the wrestler’s “never give up” slogan —was the souvenir Nick took on one particularly rough hospital trip.

“Every day he looked at it and he just knew that he would get better. He knows that if [he’s] struggling with something, just keep going,” his mom, Victoria Santillo, says. “He gets that from John Cena.”

The wrestling world has gotten a lot from John Cena, too — namely, new fans. He’s got over 37 million Facebook followers (more than Michael Jordan and LeBron James), and his recent appearance on WWE’s “Raw” drew a record 5.4 million viewers.

After the square-jawed, hulking Massachusetts native first entered the WWE in 2002, he helped knock out the brand’s so-called Attitude Era, when wrestlers were nastier and sex was a selling point.

“In the height of the Attitude Era, it could be compared to a touring frat party,” Cena, 38, tells The Post. “We made a conscious effort to clean up.”

Thanks to today’s more family-friendly image, the WWE has been able to work with more charities and advocate social causes, running anti-bullying campaigns and partnering with the pro-LGBTQ rights campaign NOH8. But Cena puts a lot of time into Make-A-Wish because, “When I hear that this child wants to hang out with [me] for the day, I think that’s the most flattering thing that I could ever be a part of.”

On a recent wish-fulfillment trip, he took a young boy to a WWE match in Washington, DC. As a surprise, during commercial breaks, Cena took the kid out of the front row, brought him into the ring, and got the crowd to chant his name.

“Man, you should have seen his eyes light up,” Cena says. “You can see the 7-year-old kid become 10 feet tall.”

Cena is 6-foot-1 and 250 pounds, but he’s about to grow into a whole new version of himself: comedy actor. He plays a clueless but sweet suitor for Amy Schumer’s character in the comedy “Trainwreck,” opening Friday.

It’s an intentional flip on traditional rom-coms, where the girl usually emotes all over her insensitive love interest. Cena lands some of the best lines, particularly during a scene where Amy encourages him to be vocal during sex.

“I’m gonna give you my full pecker!” he shouts. “I’m gonna fill you full of protein!”

“I’ve been looking to do comedy forever. What I do every Monday night is action,” Cena says, referring to the live wrestling showcase “Monday Night Raw” on USA. “[I’m] essentially a superhero trying to swoop in and save the day.”

“Trainwreck” director Judd Apatow is famous for letting his actors improvise outside of the script. Thanks to WWE’s mix of scripted entertainment and real athleticism, Cena was ready.

“What you see on-screen is 95 percent improv,” he says. “It’s kind of the way we do business in the WWE. You have a loosely defined story. As long as you play within the goal posts, the dialogue is whatever [you] make it.”

It’s hard to imagine now, but Cena’s path to the ring began because elementary-school bullies in his small town of West Newbury, Mass., weren’t fans of his choice in music.

“Man, was I about as hip-hop as you can get,” says Cena, who splits his time among California, Florida and Massachusetts. “I had a giant Kid ’n Play haircut, most of my wardrobe was rayon. I didn’t exactly look like everybody else.”

After two years of asking for a weight bench, he got one at age 12. His love of wrestling was already in place by then, as Cena’s family was one of the first in the neighborhood to get cable, specifically so his dad could watch matches.

“I didn’t grow up playing catch with my dad,” says Cena. “He’s the biggest WWE fan in the universe.”

Cena played football for four years at Springfield College, but never entertained going on to the NFL. Instead, he worked his way up through small wrestling leagues before making his debut in the WWE.

But all that tough-guy stuff only seems to have made Cena a bigger teddy bear when it comes to his charity work.

“He’s really good with the kids, he’s really patient, he listens,” says Racheal Martin, of Groton, Conn., whose 8-year-old son Robbie had his wish to meet the superstar granted in February, when the family visited a “Monday Night Raw” match. “You could tell it’s really something that he cares about.”

For Robbie, Cena’s positivity helped him get through often-brutal chemotherapy treatments.

“I just like how [Cena] has wristbands that say ‘never ever give up,’ ” Robbie says.

Even when Cena loses a match he can give kids hope, like at a recent defeat in Washington, DC.

Make A-Wish president and CEO David A. Williams recalls, “[Cena] leaned over the wall and whispered in [an] 8-year-old’s ear, telling him that sometimes we have setbacks, but you can never give up.”