Christina said exhaustion and speed may have been factors. "Boys at that age think they are invincible; James had just driven back from Auckland that day and met up with friends at rugby practice," she said.
She credits actor Cliff Curtis, who starred alongside her grandson in Dark Horse, for helping Rolleston focus on "getting back on track again".
"Cliff is James' mentor and knowing he is there for him means everything."
The award-winning actor, who has a role in Jason Statham's shark movie Meg, recently invited Rolleston and Christina on set to meet the cast and crew.
"Part of James' rehabilitation is keeping his focus on his career - we don't want depression to set in or anything like that.
"It's important for his wellbeing he is able to go on set and be with his acting friends".
The family was touched by the stream of visits from school and rugby mates while he was in hospital.
"They came with their love and open hearts. They were so well-mannered and respectful."
The producers of Rolleston's latest film, the yet-to-be-released Pork Pie - a remake of Kiwi classic Goodbye Pork Pie - also visited and showed him a trailer of the movie.
"I thought he'd be quite terrified looking at it, because the speed of the car might have brought up memories of the accident," said Christina.
"He didn't look at it too much, I think the noise was enough for him." Rolleston - who has two older brothers also raised by his Nana - is determined to jump back into acting but will need a couple of years to fully recover from the brain injury, says Christina.
At Auckland's ABI Rehabilitation centre, he will do physiotherapy, swimming and speech therapy.
He now walks with a slight limp and, his Nana said.
"His speech is good but not good enough to be in front of the camera yet but he has a strong inner drive so he'll make it.
"He said to me, 'Nan, if I have learned anything, it's to treasure life. It's a gift'."
