His Fake Prison Daddy by Clancy Nacht

When eighteen-year-old hacker Elias Stuyvesant ends up in a maximum security state prison, he’s woefully unprepared despite his time in juvie. On day one, he’s thrown in with a man known as the Santa Fe Slayer, Ambrose Hughes.
Hughes is quiet, disfigured, and weirdly urbane. Elias was so young when Hughes committed his crimes that he has only the faintest idea what Hughes is in for. However, Hughes makes clear that Elias is his ideal victim type…and there’s no one to protect Elias from the much larger man with his prison-jacked body and that hard gleam in his dark eyes.
Whoever paired them has it in for Elias; that much is obvious.
Elias is terrified of Hughes, but he soon realizes the other prisoners are worse. If Elias is going to survive, he’ll have to choose the lesser of the evils: To preserve himself, he’ll need Hughes for his Daddy. And given Hughes’s skewed morality, they’ll have to fake it till they make it.