"Ainul died late Thursday," said a police official.
Arun Kumar, director of the Patna-based Indira Gandhi Institute of Medical Sciences, where Ainul was admitted with splinter injuries to his brain, confirmed that he was dead.
"After battling for life for over five days, he succumbed to serious injuries in his brain," Kumar said.
According to police officials, Ainul was admitted to the hospital here as he was found crying in pain near the railway station toilet after the bomb exploded when he was trying to fix it or fit a timer.
Arun Kumar said that before his identity was disclosed by the police, he was believed to be a victim.
"With a splinter lodged in his brain, the chances of his recovery were very poor. He was unconscious and not responding to medication after being admitted to hospital," Kumar said.
Six people were killed and 83 injured in seven bomb blasts in Patna.
Six of the seven blasts took place Sunday at Patna's Gandhi Maidan before Bharatiya Janata Party's prime ministerial candidate Narendra Modi was to address a rally. The seventh explosion was at the railway station.