The Peats cannot be sure, but they presume that Stephen’s problems are rooted in concussions. Perhaps, like several of the dead enforcers and roughly 100 former N.F.L. players, one day he will be found to have chronic traumatic encephalopathy, the degenerative brain disease caused by repeated blows to the head.

For now, it cannot be accurately diagnosed until death, but Peat and his father worry that he has it. The symptoms often associated with C.T.E. — memory loss, depression, impulsiveness, addiction, headaches — are part of Stephen Peat’s daily life.

Walter Peat, a widower, overseeing the rebuilding of a house his son set on fire, views himself as the last line of defense for his erratic son’s uncertain future.

He recently reached out to Len Boogaard, Derek’s father, who has spent time since his son’s death in 2011 investigating his final years. Boogaard, 28 and under contract with the Rangers, was found dead of an accidental overdose of oxycodone and alcohol.

“What I’m going through is what he did before his son passed away,” Walter Peat said.

Relentless Headaches

Stephen Peat last played in the N.H.L. in 2005-6. He has spent most of the years since unable to hold a steady job and fighting off legal trouble — a couple of times from bar and street fights, and currently from accusations of theft, possession of stolen property and resisting or obstructing a peace officer. Peat says his only involvement in the latest episode was as a driver who unwittingly gave someone else a getaway ride.

The headaches, Peat said, began in earnest a year or two after his playing career ended. They often begin shortly after he awakens in the morning, and sometimes force him to stay in dark, quiet rooms, occasionally for days on end. Doctors and CT scans have been unable to diagnose the problem.