Eniola Aluko, from Thamesmead, died on Friday after sustaining gunshot wounds to the neck and chest.
Police said they were called to the scene, a car park on Hartville Road, just before 5pm.
Mr Aluko was found with critical injuries and died at the scene a short while later.
A post-mortem examination held on Sunday gave the cause of death as a gunshot wound to the neck and chest.
He is one of three men who have lost their lives to fatal attacks this weekend in the capital .
Police said on Sunday that they had made a fifth arrest over Mr Aluko's death. An 18-year-old man has been arrested on suspicion of murder and remains in police custody.
Four teenagers - one 17-year-old boy, two 16-year-old boys and one 17-year-old girl - who were arrested on Friday have all been bailed until July pending further enquiries.
Detective Inspector John Marriott, said: “I am continuing to appeal for anyone who was in the area at the time, who saw or heard anything suspicious to contact police.
“The shooting took place not far from Plumstead Railway Station and I would urge anyone who was commuting to or from this station to come forward."
