Just hours after leaving his son's death bed, a stoic Steve Miller has paid an emotional tribute to a "beautiful, brave young man with his whole life yet to be lived."

Cole Miller, 18, died of massive brain injuries in the Royal Brisbane and Women's Hospital on Monday, just more than 24 hours after an allegedly random coward punch in an inner Brisbane nightclub precinct.

The promising young water polo player was walking through Fortitude Valley's Chinatown Mall with a friend about 3.55am Sunday when he was allegedly felled by a single, unprovoked punch to the head.

He was rushed to hospital in a critical condition but did not regain consciousness.