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Griffin Lee, 17, was taken to Waikato Hospital's intensive care unit in a critical condition after the crash north of Tokoroa on Tuesday.
Police said he died in hospital earlier today.
Warren Lee, 53, Aesoon Lee, 52, and their daughter Julia Lee, 20, died in the crash.
Mr and Mrs Lee were American citizens living in Hong Kong, while Ms Lee had been living and studying in Massachusetts. She was due to start studying at the University of Auckland a few days after the crash.
A hospital spokeswoman last week said Griffin Lee -- a front-seat passenger -- was not aware the crash had killed his family.
The family had come to New Zealand to help Ms Lee settle into her new home before they returned to Hong Kong. She was driving at the time of the crash.
The death come amid a spate of serious and fatal crashes involving foreign drivers, prompting calls for more to be done to reduce the number of crashes involving tourists.