Judges at the Court of Appeal have upheld a sentence given to Lewis Gill, who was found guilty of manslaughter after punching Andrew Young, 40, in an unprovoked attack.
Mr Young later died from a head injury which he suffered when he hit the ground.
Mr Gill, 20, was sentenced to four years in jail for the offence, a decision which provoked outrage.
Mr Young's mother, 71-year-old Pamela Young, called the sentence "an absolute joke".
The incident occurred on November 6 last year outside the Tesco Metro store on Charminster Road in Bournemouth.
Gill later pleaded guilty to manslaughter and was sentenced at Salisbury Crown Court.
Attorney General Dominic Grieve argued today that the sentence was not a proper reflection of the gravity of what Gill did.
But the Appeal Court judges decided there should not be any change to the four years being served by Mr Gill.
Read: Mother's outrage at four-year sentence for man whose single punch killed her son
The Court of Appeal has ruled that the four-year sentence given to Lewis Gill, 21, for the one-punch manslaughter of Andrew Young was not "unduly lenient".
A four-year jail sentence given to a man who killed an Asperger's sufferer with a single punch in an unprovoked attack in Bournemouth is to be reviewed by leading judges today.
Andrew Young who was 40, died after being struck by Lewis Gill on November 6 last year.
Three Court of Appeal judges in London will decide on whether the four years for an offence of manslaughter, is "unduly lenient".
The sentence, imposed in February at Salisbury Crown Court, has been referred to the appeal court by Attorney General Dominic Grieve, who will be presenting the case.
The incident happened outside a Tesco Metro store in Charminster Road, Bournemouth. Mr Young suffered a serious head injury and later died in Southampton Hospital.