He constantly has jaundice, itchy skin, he has trouble eating and sleeping and has to be tube fed at night. He had to use a nasal tube in his first year because he couldn't put any weight on.
"It's been a huge weight on the family," Ms Christie said. "The last year and a half have been very hard and stressful."
However, things recently got a little easier for the family. They received a lower two-year fixed home lending rate of 2.42 per cent thanks to ASB Auckland Marathon Run Down Your Rate runner Katie Kemp
The reduced interest rate will have a visible impact on the family who were considering selling their house to reduce their financial burden with ongoing medical costs.
When Ms Christie told her fiance, Andrew, he nearly cried.
"It is absolutely life changing - reducing the financial strain on our family while I'm off work caring for Carter, as well as being able to look forward to being mortgage free sooner - it is truly incredible," Ms Christie said.
Ms Kemp said when she met Carter at the finish line he stretched out his arms to hug her.
"He's such a cool little dude and he coped so well with all the cameras.
"They really seem like a lovely family so it feels amazing to be able to help them."
Ms Kemp has three children of her own and the story really touched her heart.
"It's like the gift that keeps on giving, first I won the race and now I get to help this wonderful family."
Ms Kemp said it was very special to be able to do something she loves to benefit someone else.
Ms Christie and Carter were able to be at the finishing line on November 1 because they are long-stay guests at Ronald McDonald House.
Carter is recovering from a liver transplant he received in August.
Ms Christie has seen incredible improvement since the operation.
"He started eating where before he could only have special formula. He is walking and has so much more energy. He's been reaching huge milestones in the last three months," Ms Christie said.
"He's a bit of a ladies man, he's very popular with all the nurses."
While the progress is encouraging the family are aware of the battle ahead.
"We expect Carter will have ongoing medical procedures for the rest of his life."
The severity of the disorder can vary within the same family, with symptoms ranging from so mild as to go unnoticed to severe heart and/or liver disease requiring transplant.
Ms Christie's eldest daughter also has the condition, however, suffers only minor symptoms.
It had been tough for the mother and son being apart from the rest of the family.
Mr Sim and their two daughters, Mya, 12, and Lily, 4, were still in Napier.
Ms Christie had had to give up her job to care for Carter, and the family still had bills to pay.
Although the lower interest on the mortgage would help them stay afloat.
If you wish to contribute to Carter's cause please visit his givealittle page. https://givealittle.co.nz/cause/littlecartersfightwithalagilles/
