Published Wednesday, 10 October 2012
Wee Oscar travelled to America for treatment. (© Family photo)
The four-year-old and his family travelled to Philadelphia over the weekend in the hope of beginning six months of the specialist treatment, but he has since been admitted to intensive care.
Tests have now confirmed Oscar is not suitable to receive it because of an issue which was discovered with his heart.
The family said once he can be taken out of intensive care they will return to Northern Ireland.
Little Oscar has stage four neuroblastoma, which causes tumours to develop, in the stomach, spine, chest and neck, but can also spread to other parts of the body.
It is an aggressive childhood cancer which could kill him.
Oscar's family were able to make the journey to Philadelphia after they managed to raise £250,000 towards his medical costs with help from a Twitter campaign which went global.
Almost 16,000 people are now following their story via the #TeamOscar hashtag, including many famous faces such as Belfast boxer Paddy Barnes, Snow Patrol frontman Gary Lightbody, Celtic star Joe Ledley and others.
Before departing, the Knox family told UTV they want to try and give their brave boy "a chance at life", and said they will not to stop fighting to save him.
Stephen Knox said: "We just want him to have a chance at life, to grow up.
"We could sit back here at home and cross our fingers, and hope to never see it again but that's not what we want to do."
Leona Knox added: "It's been really, really tough on him and he doesn't understand and we don't have any answers.
"But he's managed to get through it so well. He's almost showing us how to do it."