His parents were told a test after the blood had gone would solve the mystery. But the Starship's radiology and neurosurgery experts have disagreed over the case, and yesterday sat down to analyse an MRI scan.
"In that respect, we are back to square one over what happened to him - we don't know why he had a brain bleed that resulted in a stroke," Ms Thomas said.
More worryingly, it posed the question of whether Lewis' brother Cohan could also be at risk.
"We are really concerned, having identical twins with the same DNA, that it could happen to Cohan as well," she said.
The couple's stress has been eased by the courage of their son.
"He's probably the most inspirational person Andrew and I have ever met ... he's the one who has kept us motivated and going through this horrible situation, he's remained so positive this whole time," she said.
"And we had the best Christmas we've all ever had, as there was a while where we didn't think the four of us would be home."
The family had also been left "completely astonished" at the generosity of their community and others around the country - including many Weekend Herald readers who rallied around Lewis when his plight was publicised in November.
At that stage, the family had exhausted their savings and faced paying an extra $500 each week for the crucial four-weekly rehabilitation sessions their son needs.
But donations from readers and more than $5000 raised at a major garage sale - where locals dropped off goods ranging from tyres to a pallet packed with manchester and other items - have ensured Lewis intensive rehabilitation for at least another year.
"We just want to say thank you to the New Zealand public. People have been amazing and the support we've received has just made us feel so proud to be New Zealanders."
And while specialists in a fortnight's time will lay out the road ahead for Lewis, the signs he has so far shown are promising.
"At first we thought we would have to get him in to some kind of school, but he started pre-school last week, and everyone agrees he's gone ahead in leaps and bounds.
"He's trying to run and get back on his bike again and people can understand him when he talks."
Readers can leave messages of support for Lewis and his family on his Facebook page.