Skyler Cassity’s football career ended in an Atlanta doctor’s office during a routine physical before his junior year of high school.

He was born with pectus excavatum, a common condition where the breastbone sinks into the chest, causing a concave appearance. Usually, it’s harmless. But Cassity’s worsened during his growth spurt so his heart was now wedged between his sternum and spine. He needed to have surgery and quit football. Doctor’s orders.

Cassity quit playing but didn’t quit the game. His Riverwood International Charter School coach, Robert Ingram, gave his 16-year-old quarterback a coaches’ headset. He hasn’t taken it off since.

In January, Sam Houston made Cassity the youngest defensive coordinator in the FBS. But the 29-year-old has a decade-plus of coaching experience. And his playbook predates him. His father, Mike Cassity, was a Division I defensive coordinator for 22 years.

In elementary school, while Mike coached at Louisville, Skyler used to sit in his father’s office and pass the practice time playing the NCAA Football video game and drawing up plays. He’d hand off his best work to Mike and ask him to show it to head coach Bobby Petrino. Once, he crafted a fake toss sweep with a throw back to the X receiver running a go route. Louisville ran a similar version during a nationally televised game. Mike convinced Skyler that it was his play.

A coach's son’s first coaching thrill.

Football connected Mike and Skyler. Most nights, Dad returned from the office after Son went to bed. But Thursday was family night for the Cardinals’ staff. Mike sat in his bedroom watching two games of the upcoming opponent, making his defensive call out loud. Skyler would crack the door, invite himself into the room and prop himself on the bed.

Mike asked Skyler what personnel the offense was in before every play. Twelve (one running back and two tight ends), Dad! Or Eleven (one running back, one tight end). This one’s Twenty-One (two running backs, one tight end). By the time Saturday came around, Skyler was lecturing fans in the stands on what formations the opponent would line up in.

No wonder Ingram made him a student coach after his career ended.

Skyler watched his father work 80-hour weeks, coaching for nine different Division I teams in just over two decades.

“He knows the grind that coaches put in,” Mike said. “One day you’re a hero, and the next day they’re firing you. He saw me hired, fired, re-hired - you name it.”

And Skyler wanted all of it.

He attended nearby Auburn and started as a freshman on the lacrosse team. Football, however, brought him to the campus. The Tigers were coming off the Cam Newton-led title run, part of seven consecutive national champions from the SEC. He wanted to get involved in college football at the highest level.

Skyler trekked to Auburn’s football office and met with Casey Woods, SMU’s current offensive coordinator, who was then in charge of recruiting. Woods, the son of longtime college coach Sparky Woods, saw a fellow coach’s kid asking for any work possible to get his foot in the door. As a matter of fact, he did have work. Grunt work.

Skyler was in charge of unsolicited emails to the football department. He’d arrive at the office before Woods and sift through hundreds. By the time Woods started his work day, 85 of the important emails had been opened up, along with recruiting services such as Rivals and 247Sports where Skyler researched players. On the weekends during recruits’ official visits, he’d hand out brochures and show people around.

“No job was too small. There was no ego in it,” Woods said. “He just did exactly what I asked him to do. He was very coachable. He was trying to get in. I think he took on the mindset that there’s a cost to what it costs.”

There’s a misconception that coaches’ kids get a free ride. Skyler paid his dues.

Skyler constantly asked Woods if he could sit in on team meetings, and Woods constantly told Skyler he needed to get his assignments done first. Skyler worked so hard Woods couldn’t say no.

Skyler earned the opportunity to sit in the back of the running back position meetings. That evolved into coaching Cameron Artis-Payne and Kamryn Pettway one-on-one as a college student.

He credits Woods and Auburn for launching his coaching career. Years later, Woods says Skyler motivated him during an inflection point in his life. He was serving in an off-field role at Auburn after a stint as the wide receivers coach at Arkansas State. Woods wanted another on-field opportunity. Skyler reminded him of the motor it’d take to get it.

Sometimes, the student teaches the master.

“I was trying to get back into coaching when I was at Auburn,” Woods said.  “It was fun watching his work ethic and his attitude coming in there and doing those things. He was a refreshing reminder for me.”

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Mike Cassity sat in a doctor’s office in Little Rock, Arkansas, when a woman approached him and asked if he by any chance had a son that coached at Missouri State.

Sure did. His boy, Skyler, was the outside linebackers coach. Bobby Petrino hired him to a full-time role nearly two decades after he drew up trick plays for the coach in elementary school.

Well, the woman said, her son had attended Missouri State’s youth camp, and Skyler was so engaging with the kids while he ran it.

Mike took pride in this compliment. He’d told Skyler, whether you were a graduate assistant or a full-time coach, there’s always jobs around a program coaches don’t want to do because they’re too busy. Or too good for it. Mike had a smile on his face as he finished his cancer treatment.

Mike was at the height of his coaching career in November 2012, the defensive coordinator at Kentucky, when he pulled his hamstring sprinting down the field with the kickoff unit. A doctor’s visit revealed a tiny black dot on his third vertebrae. Multiple myeloma. Incurable bone marrow cancer.

Skyler was finishing high school when his father came out of the hospital after a month and a half. He was now extremely aware of the fragility of life at a moment when most feel invincible. His football career ending was a blow - but this was life or death.

Skyler and Mike both responded to their diagnoses by coaching. This profession is a microcosm of life.

“In football, you can be at the highest high, then have a loss and be at the lowest low,” Mike said. “You can feel sorry or grovel about yourself, or fight for another day. That’s what coaching is all about.”

Every morning, Mike told Skyler and his brothers two things. One - the only thing in life you have control of is your attitude. Two - we all have an hourglass in our body in which we don’t know how much sand is left. Every morning is the first day of the rest of your life. So what are you going to do about it?

Doctors told Mike he had enough sand for six years. He’s stretched it to 12.

Skyler attacked every day with his father’s teaching in mind while an Auburn student, then climbed his way through graduate assistant jobs at Texas State and Texas Tech. The intern who sifted through emails became the full-time coach running kids’ camp.

“He’s a guy that didn’t take any shortcuts,” Casey Woods said.

And he still became an FBS defensive coordinator in record time.