Kevin Federline claims his and Britney Spears‘ son Sean Preston was “terrified” to stay with his mom during some of their most turbulent years.

In his memoir, “You Thought You Knew,” Federline recalls gut-wrenching nights comforting the boy, who he says would cry and cling to him whenever it was time to visit Britney.

“Every time Preston had to go to his mom’s, they had to pry him off my neck,” Kevin reads in the audiobook version. “He’d cry, scream, and hold on with everything he had.”

“Even the slightest movement would wake him in a panic,” Kevin writes. “I’d sit beside his bed, my finger resting on his arm, and the second I moved, he’d jolt awake, crying for me.”

Federline says he eventually turned to professional help … a therapist who “quickly confirmed what I already feared: this wasn’t just separation anxiety. Preston was showing signs of early trauma, even as a toddler.”

Kevin claims Preston “didn’t feel safe at his mom’s” and “felt neglected while Jayden received most of her attention.”

Britney’s rep previously told TMZ … “With news from Kevin’s book breaking, once again he and others are profiting off her and sadly it comes after child support has ended with Kevin. All she cares about are her kids, Sean Preston and Jayden James and their well-being during this sensationalism. She detailed her journey in her memoir.”

The former backup dancer shares Preston and Jayden with the singer. Britney lost custody of the boys in 2008 after a string of personal and legal troubles that led to her 13-year conservatorship. Over the years, Kevin has maintained primary physical custody, while Britney’s contact with her sons has fluctuated.