According to Good Morning America, Wyatt Haas felt sick and suffered headaches, leading to diagnosis of medulloblastoma in November.
A day later, the kindergartener underwent an operation to remove part of the tumour in his brain.
As St Jude's Children's Research Hospital in Memphis, Tennessee, gave approval to treat him further, the family prepared to leave their home in Fallon, Montana.
Before the family's scheduled departure earlier this month, Jennifer Nielsen, one of Wyatt's classmates, learnt about how much the boy loved unicorns.
So she set up a 'unicorn ride' for the boy. She used washable animal chalk to turn one of her horses into a colourful unicorn and surprised Wyatt at a local park where his classmates and teacher waited to send him off.
A pony owned by her friend was also roped in for the fun party.
"I don't think he expected to see a unicorn at the park," his father Zach Haas said.
"I think he just expected to see his friends. He was super excited," he added.
A post by the family on Facebook read: "We moved into the most amazing community and we seriously could not be more grateful!
"We're going to miss home so much. So glad to have these pictures to look back on for Wyatt!"
Wyatt and his mother left for Memphis on December 6.
Another post on their Facebook page on Wednesday informed followers that Wyatt had a second brain surgery to remove more tumour.
"We are confident that things will go good, and hopefully Wyatt won't have the neurological issues he had with the last resection as they are just cleaning up what's left," the post read.
"Keep him in your thoughts and prayers, and we will update later to let everyone know how things went," it added.
A GoFundMe set up to help the family with medical expenses has so far raised more than $10,000.
( With inputs from IANS )