CCTV footage, revealed in court, shows Andrew Young falling into the road and not moving after a sudden blow to the face from Lewis Gill.
Shockingly the 20-year-old attacker will be freed in around two years – a move that has been greeted with outrage from Mr Young’s heartbroken family.
Seconds before he was fatally struck, Mr Young had been involved in an argument with Gill’s friend Victor Ibitoye, who he told not to cycle on the pavement.
Yob Gill then took offence and punched him once in the face before walking away without a care for his victim.
The unprovoked attack happened in broad daylight and came after a row about cycling [IG]
Mr Young, who suffered from Aspergers Syndrome, was rushed to hospital in Bournemouth but died the following day from head injuries, with his mother Pamela at his side.
Gill admitted one charge of manslaughter at Salisbury Crown Court and was jailed for just four-and-a-half years.
Today Mr Young’s mother reacted with fury, saying: "I sat with him when he died. I wish that awful man who took my son away had pleaded not guilty so he would have got a longer sentence.
"The sentence is an absolute joke. I'm a committed Christian but I think that if someone takes a life they should be prepared to forfeit their own."
Andrew Young died in hospital the day after the attack with his mother by his side [BNPS]
The attack happened at 4.25pm on November 6 last year outside a Tesco Express in Charminster, Bournemouth.
Mr Young was able to speak several language but had the social skills of a 14-year-old because he suffered from Aspergers Syndrome.
The court heard Mr Young told Mr Ibitoye during the argument that riding his bike on the pavement was a 'dangerous activity'.
Mrs Young said: "He was very particular because of his Asperger's and he wouldn't have liked seeing someone riding a bicycle on the pavement because it was dangerous."
She said her son often attended Sunday mass at the St Edmund Champion Church and was a member of the naval section of the Combined Cadet Force.
Pamela Young blasted the sentence handed to Lewis Gill describing it as a joke [BNPS]
Taxi Driver Simon Scott, who knew Mr Young for six years, said he "wouldn't hurt a fly".
He said: "Andrew was always friendly and polite and often struck up conversations with strangers at the bus stop near the taxi rank.
"Andrew was gentle and wouldn't hurt a fly."
A post-mortem revealed that Mr Young had no injuries to indicate he tried to defend himself.
The court heard Mr Ibitoye did not know what Gill was about to do and cycled away from Mr Young after the argument.
Prosecutor Kerry Maylin said: "The defendant claims he was worried about Andrew Young because he thought he was being menacing.
"He said Andrew Young put his hand to his pocket and he thought he was going to pull out a knife or a gun."
Lewis Gill was jailed for four-and-a-half years for the attack on Andrew Young [BNPS]
The court heard Gill punched Mr Young after he allegedly made a racist remark.
Judge Keith Cutler said Mr Young did not represent a threat to Gill.
He said: "You are a powerfully built young man.
"You must have known that it was going to cause a significant injury and, very sadly, it did."
Gill was also sentenced last Friday to two, three months sentences for the attack while on a suspended sentence for robbery and for handling stolen goods.
The four jail term along with the two three month sentences will run consecutively.
