Mr Preston, who is in New Zealand following his mother's funeral, said Carter's recovery was looking "reasonably good'' but the long-term effects of his injuries would not be known until he was about 6-months-old.
"Then you can start doing some reflex tests and cognitive tests on him. We'll bring him back to New Zealand at that stage, hopefully, and get some specialists onto it.''
Mr Preston will return to Vietnam this week to be with Carter and his partner Chau Nguyen.
"I really want to get back and get life back to normal again as quickly as possible.''
Ms Nguyen had the support of family in Ho Chi Minh City and had handled the ordeal "in the best possible way you could imagine''.
A trust fund set up in Ms Ferne's memory has so far raised almost $60,000 for much-needed medical equipment for Ho Chi Minh Children's Hospital.
Mr Preston would liaise with the hospital when he returned to Vietnam to find out what equipment it needed and arrange delivery.