Nathan Cartwright is the fourth person to have died after the horror crash in Bromley Lane, which happened at about 8.50pm on Tuesday.
He was taken to hospital in a critical condition but died on Saturday.
In a loving tribute released today, his mother said: "My gorgeous boy Nath taken way too soon. You lived your short life to the full. You are at peace now.
"I would also like to thank the emergency services and the QE Hospital for saving his life and giving me the chance to say goodbye to him."
Nathan was one of five people in a yellow Skoda that crashed into the tree.
The driver of the yellow Skoda Fabia, Joshua Parkes, 21, and rear seat passenger Lucy Tibbetts, 16, were pronounced dead at the scene.
Another 16-year-old girl also died in the collision. Her family do not wish to release her name at this stage and have asked for their privacy to be respected.
A fifth person – also a 16-year-old girl – remains in hospital in a stable condition.
Specialist officers continue to support the families of all five victims.