A teenager is in hospital with serious brain injuries after he was coward punched by a stranger outside a popular Adelaide nightclub.

The victim’s parents say they thought they’d lost their boy and are now begging his attacker to hand himself in.

The attack on Beau Brownbill, 18, happened at 1am today on Hindley Street.

READ MORE: PM makes ‘downpayment’ on $30 billion shipyard to build nuclear submarines

Beau Brownbill, 18, is in hospital with brain injuries. (Nine)

His mother Megan said it was “every parent’s worst nightmare.”

He was seen exchanging words outside Fabric nightclub with a man whoe left the queue to confront Beau.

A scuffle followed and then one punch.

Beau was left lying on the concrete as the man fled.

“I want that gentleman to understand what he’s done is, he’s changed someone’s life forever, he’s changed our family’s forever,” father Tony said.

The attack on Beau Brownbill, 18, happened at 1am today on Hindley Street, Adelaide. (Nine)

READ MORE: Man critical after crashing e-bike while allegedly fleeing police

Beau was knocked unconscious and raced to hospital where he remains with a fractured skull and bleeding on his brain.

“He’s at the time of his life where he should be experiencing the best things in life, and this should not be happening,” Megan said.

Beau’s parents were told to prepare for the worst.

“They took us into a side room and we thought it was it for Beau and that was the end for Beau so it was very very distressing,” she said.

Beau Brownbill’s parents say they thought they’d lost their boy and are now begging his attacker to hand himself in. (Nine)

While there have been encouraging signs in Beau’s recovery, the true extent of his brain damage remains unknown and only time will reveal what lies ahead.

Beau’s attacker is a stranger he says he’d only met earlier in the night.

No arrests have been made.

“You’ll get some pain in regards to what the law inflicts on you, take it on the chin like you gave it to my son on the chin,” Megan said.