Former NFL head coach and influential defensive coordinator Buddy Ryan is again battling cancer, his son Rex told the New York Daily News on Tuesday.

Rex Ryan, the coach of the New York Jets, didn't reveal what type of cancer his father has, but said it wasn't melanoma, a type of skin cancer that Buddy Ryan has beaten twice before.

"It's going to take more than this to get rid of him," Rex Ryan told the Daily News. "That's what he always says."

Ryan said his wife, Michelle, was with his father Tuesday at Memorial Sloan-Kettering Cancer Center in New York City, where he was receiving a second opinion.

"He's 80 years old [Buddy Ryan is actually 77], but he's doing great. He's tougher than s---," Ryan told the newspaper.

Ryan grew up watching his dad -- a Korean War veteran who turned to football after graduating from Oklahoma State -- build some of football's top defenses behind a relentlessness that focused on creating havoc on the field.

Rex Ryan, left, and Rob Ryan, far right, have continued their father Buddy Ryan's legacy of coaching tough NFL defenses. Al Pereira/New York Jets/Getty Images

He was on the defensive staff of the Super Bowl-winning Jets in Super Bowl III in 1968. The Chicago Bears carried Ryan off the field next to coach Mike Ditka after Ryan's vaunted "46 Defense" helped the Bears destroy the New England Patriots 46-10 in Super Bowl XX.

He later was head coach of the Philadelphia Eagles and Arizona Cardinals. Rex was on his dad's staff for two years with the Cardinals in the 1990s.

Besides his legendary stay in Chicago, Buddy Ryan also was a defensive coordinator with the Minnesota Vikings and Houston Oilers. His stay in Houston was tarnished when the then-59-year-old Ryan took a swing at then-offensive coordinator Kevin Gilbride on the sideline during a game.

Rex Ryan's brother, Rob, is entering his first season as defensive coordinator for the Dallas Cowboys. Rex said his father is still getting used to his son coaching for one of his biggest rivals while he was coaching in Philadelphia.

Buddy is "struggling with it a little bit. ... He looked at Rob like he's Benedict Arnold or something," Rex said Tuesday.

Despite their dad's wishes that they pursue another profession, the Ryans followed him into coaching. When Buddy Ryan, whose "46" scheme changed the way defense is played in the NFL, coached in Philadelphia, he urged his sons to accept jobs with a food service business at the airport.

Buddy Ryan learned the hard way that the grueling hours and constant travel would take a toll on his personal life. He and his wife, Doris, split when the boys were young. He wanted his sons to follow a different path.

"They didn't listen," Buddy said last October before Rex's Jets played Rob's Browns. Rob Ryan was the defensive coordinator in Cleveland before being hired by Dallas this offseason.

Information from The Associated Press was used in this report.