Ultrarunner Jason Romero is legally blind, suffering from a degenerative eye disease called Retinitis Pigmentosa that gives him tunnel vision. He describes it like looking through tubes roughly the diameter of a quarter.

The conditions causes Romero particular challenges as a runner. At night or on unfamiliar terrain while training or competing in one of his latest endurance feats—which have included the Badwater 135, Leadville Trail 100, and a trans-America run earlier this year (the first visually impaired athlete to do so)—he uses a guide. Typically, it's a buddy or someone familiar with the terrain.

Recently though, Romero has found a new person to fill the role: his 15-year-old son, Sage.

“Sage and I will go out for training runs and he will be the eyes of the operation,” Romero told Runner’s World by phone. “I can’t see the walk signs at intersections, so he will stop me when we can’t go or let me know there is a curb coming up.”

The partnership started four months ago on a scorching summer day at Northfield High School in Denver, Colorado.

Romero wanted Sage, who was born with autism, to get invovled with an afterschool activity to meet and socialize with his peers as a new freshman. Romero asked Northfield's cross-country coach, Patrick Thornton, if Sage could help at practice as the team’s manager. Sage had always loved sports, and he even played with the flag football team in middle school, so Thornton happily allowed Sage to help out.

On the first day of practice, Thornton handed Sage a clipboard with a sheet of names to take attendance before the dozens of wiry teenagers loped off on a warmup lap.

Romero attended the practice with Sage, who dutifully crossed each name off and watched the throng of runners start on the one-third-mile loop around the building. But shortly after they took off, Sage threw the clipboard down and ran to catch up.

The 46-year-old father of three was shocked. “I remember thinking in that moment, ‘Well, I guess he is going to be an athlete,’” Romero said . It was the first time Sage had shown any interest in running.

Romero, who can navigate during the day with his limited vision, ran to catch up to Sage during the short loop. Sage’s face was red, and he was breathing heavily. But he finished the lap, and Sage became an official member of the team.

In order to teach his son pacing and hydration, Romero started running with Sage during practices. Sage would warn about unexpected obstacles like poles and curbs, grabbing his dad’s arm before a collision.

“After two weeks, he was able to run and walk a couple miles,” Romero said. Sage then competed in his first cross-country meet, a 5K, with his dad by his side. The duo finished in last, but they were greeted by his coach and the entire Northfield Cross Country high school team.

“Everyone was just ecstatic,” Romero said.

Over the past four months, up to four times a week, Romero and Sage would run together. Romero said the time alone has helped him get closer to his son.

During longer runs up to five miles, Sage will replay scenes from his favorite movies like Aladdin and Toy Story in his head. He’ll mutter the lines and laugh when he remembers a funny scene. When the training gets tough, he will say to himself “oh boy,” and trudge forward.

After a month and a half of consistent mileage, the duo went for a run on a trail near their house. It was an out-and-back loop, and Romero asked Sage to let him know when he wanted to turn around. They made it 4.5 miles before Sage was ready to head home. The nine-mile run convinced Romero that Sage could run a half marathon. He asked his son if he wanted to do the Rock ’n’ Roll Denver Half in October.

Sage readily agreed. The morning of the race, the duo was given permission to start early with other disabled athletes because Sage typically gets distressed around crowds. But before the gun sounded, he told Romero he wanted to start in the corral.

They finished in 3:19:15.

“Near mile 13 Sage started dancing when he heard a band, then before the finish line he took off and dusted me,” Romero said.

He said he’s seen a dramatic change in his son since they started running together. For one thing, Sage is more confident.

“I have heard him say, ‘Oh dad, I am really proud,’” Romero said. “That is just amazing, I have never heard him say that before.”

Romero and Sage are training through the winter for next year’s cross-country season, but they have bigger plans on the horizon. Romero hopes they will run a marathon together before Sage graduates high school.

“For me just as a person, it is so inspiring to see what Sage is doing,” Romero said. “But as his father, I am so proud I am practically bouncing out of my skin.”

This article originally appeared on RunnersWorld.com.