Kevin Williams tried to hang himself in the grounds of Buckler's Mead School in Yeovil in 2003.
Just hours before he discharged himself from Yeovil District Hospital where he was being treated for an overdose of amphetamines and alcohol.
The hanging left Kevin severely brain damaged and he had to be fed through a tube.
He spent the next three-and-a-half years in care homes and hospital and developed a series of chest infections.
But in June 2007 a decision was made to stop giving Kevin antibiotics and he died in Salisbury Hospital days later, aged 27.
His dad, Stephen, 53, of St Michael's Avenue, said an independent person was brought in to act as an advocate for Kevin under the recently introduced Mental Capacity Act.
Mr Williams said that he himself should have been allowed to decide what was best for his son. He had spent nearly every day with Kevin since the hanging.
"I knew more about that boy than any of those doctors knew," he said. He is still waiting for an inquest to examine the circumstances of the death and is concerned he may run out of time to mount legal action, which has to be started within three years.
Mr Williams, a self-employed roofer, is critical of the Mental Capacity Act 2005 which sets out how decisions can be made for people who cannot make their own.
"My main concern is to identify the part that the advocate played in allowing Kevin to die," Mr Williams said.
"It will be three years in June since he died and I don't have any dates for an inquest or any answers."
A former Buckler's Mead student, Kevin would have been 30 last Friday. He was the eldest of three children.
His dad said: "We still grieve, nearly three years later. We want the closure from an inquest. A death like this, especially of a child, is something I wouldn't wish on my worst enemy. I am still very angry. I need the truth to come out."
Paul Tranter, Wiltshire coroner's officer, said: "We are keen to get the inquest out of the way before there is any anniversary. We are mindful of the fact that it is close to the third anniversary."