His hair is falling out in clumps and he is easily confused about who has been to visit him in the hospital wing which he shares with other boys considered "at risk".
Nervously seated in an armchair in her sitting room, Mrs Pasquale describes the agonising wait her family faces to find out if William has suffered brain damage.
Mrs Pasquale, 39, has been separated from her husband for some while. Her only daughter lives away from home in Manchester.
She has agreed for her son to be identified by the Independent on Sunday because she wants to prevent a similar incident happening again.
"He's just a big, gentle giant," she says. "Everyone loved him and he always had friends coming round to visit but now he won't even lift his head up. We still don't really know what happened."
William has problems reading and was receiving special needs tuition but his teachers felt he needed one-to-one tuition in a new school.
"He was off school for a while while they were sorting his education out. This meant he got influenced by older boys who used to take him out in cars," she said.
"I didn't go with him to court when he was sentenced but the solicitor said William just looked at him and said he wanted to go home."
At 4am the next day, Jayne was woken by the sound of the phone ringing and sensed it had to be bad news.
"There was a policeman on the phone and he told me what had happened," she recalled. "I could not believe it because William had never tried anything like that before. They said to get there quickly because his condition was deteriorating."
It took nearly three hours by train, underground and taxi for Jayne to get to the hospital.
"All he wanted to do was to be with us," she said.
"As soon as he went back to prison we were only allowed to visit him for an hour or two."
William is not the first boy to try and kill himself at Feltham. There have been four others who succeeded.
The youngest of the four was Jeffrey Horler, only 15. His home was in Norfolk, but he was sent to Feltham, 200 miles away, because no local institution could take a boy of his age.
Another, 18-year-old Lee Waite committed suicide just hours after he was brutally assaulted by other inmates.
Johnny Cash, 19, had tried to kill himself before. He hanged himself in a punishment "strip cell". His last letter complained about the police, the cold and the boredom of being locked up 23 hours a day.
John Kirkland, 20, was given a single cell at Feltham, despite a history of depression which had led prison doctors to put him in the highest category of suicide risk.
Unlike the mothers of these four boys, Jayne knows there will be a time when William comes home to see his dog, Jet. But she worries for his safety and is angry he has not been allowed to come home sooner.
"I was lucky not to lose him," she says.
"But he is being treated like a mass murderer - how can you do that to a child?"Reuse content