The charismatic 5-year-old started a viral sensation in March when thousands of people from all over the world participated in the "Wear Yellow for Seth" campaign to cheer him up.
His parents posted a simple video of him holding up signs to explain his condition, severe combined immunodeficiency, which means he has no immune system and must live in an extremely clean environment.
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In the video, which has been viewed more than half a million times on YouTube, Seth asked everyone to wear his favorite color, yellow, on March 27 to show support.
Seth's mom, Leanne, told ABC the family was overwhelmed by the support from around the world, and it has continued to help Seth stay strong through the tests, surgeries and even this weekend's transplant.
"It really helps us stay positive for Seth and when Seth is well enough he is looking at the photos and it's making him smile," she explained. "We very grateful for everything Seth's supporters have done. It has been amazing."
Seth's journey, though, is far from over. He has undergone chemotherapy in order to prepare for the transplant, and his mom has been explaining all of the side effects of that, including mucasitis, bleeding of the mouth, on the family's blog. Leanne told ABC that for now Seth is getting a lot of sleep, which is good.
And what about that winning smile that won over hearts around the world? Despite everything he has been through, Seth continues to show it as best he can, Leanne said.
"His cheeky happy personality is still there but it's much more subdued," she said. "We can't wait for it to come back fully."
Friday Seth was the recipient of a bone marrow transplant using cells from his father, Nik. The family is hopeful that these cells will help Seth build his own immune system, but they won't know for a few weeks whether it was successful. Leanne said she was worried but optimistic that the transplant will work.
"After that he's not out of the woods," she said, "but we should have found the path to lead us out!"
Read more about Seth on the family's blog and on his Facebook page.