William Maguire, 54, was put in “an almost impossible position” by his father’s illness, and helped him to commit suicide “out of great love”, assistant coroner John Matthews told Isle of Wight Coroner’s Court.
Mr Maguire Jnr was arrested by Hampshire Police but later released without charge after the CPS decided not to go ahead with the prosecution.
His father, also called William Maguire, had a series of health problems including severe lung disease, raised blood pressure, bowel inflammation and depression. He was then diagnosed with motor neurone disease at the start of 2015.
He had talked about travelling to Swiss assisted suicide clinic Dignitas and was “determined” to end his own life, the inquest heard.
He was found dead at the wheel of his car parked in his garage in Cowes, Isle of Wight on March 1 2015 after his son called the police.
Pathologist Dr Basil Purdue said he died from a mix of drug intoxication and alcohol.
His son had helped him into the car and left him with drugs and a bottle of whisky, the inquest heard.
Mr Matthews said: “It’s an extremely depressing condition not only for the sufferer, but for the family of the sufferer. I’m not in the least bit surprised Mr Maguire acted in the way he did.
“I’m satisfied that on March 1 he was quite determined to be successful in his suicide attempt, so a belt and braces operation was undertaken by him using the somewhat resistant assistance of his son.
“He put his son in an almost impossible position, in my view.”
Mr Maguire Jnr was arrested on suspicion of assisting a suicide but was not prosecuted. After initially denying involvement he later said: “You already know. I helped my dad commit suicide.”
He also told officers that he “loved his father very much”.
A spokesman for the CPS said: “We were passed a file in relation to this case, but it was decided that there should be no further action as it was not in the public interest to proceed.”
CPS guidelines say that any prosecution for the crime of “encouraging or assisting suicide” must be in the public interest.
Prosecution is less likely if the victim had already made up his mind about killing himself, help from the suspect was minor and reluctant and the suspect reported the suicide to police.
Sarah Wootton, chief executive of Dignity in Dying, which campaigns for assisted dying, said: “Tragically, this is not an isolated incident.
“Around 300 terminally ill people end their own lives in this country every year. Every eight days someone from Britain travels to Dignitas for an assisted death.
“The UK’s current blanket ban on assisted dying denies dying people the choice and control they deserve at the end of life.
“It forces many people like William to take matters into their own hands, ending their own lives behind closed doors in traumatic circumstances. The effect this has on their loved ones can be devastating.”