The child, now 12 – who cannot be named for legal reasons – was going home from Springburn Leisure Centre in Glasgow when he was attacked.
Giving evidence from behind a screen, the boy told the jury he was diagnosed with leukemia in 2009 and was still receiving chemotherapy and medication in 2011.
He said he was crouched down, feeling ill, when his friends shouted at him to run.
The boy, from Somalia, started running and he said he heard "swearing and racist things" being shouted at him, then he stopped.
He said he turned towards the man who was running at him and that is all he remembers.
He said: "That's all I saw, him run at me", adding "my friends told me he headbutted me".
Procurator fiscal depute Natalie Henderson asked why his family moved to London. He said: "I was having nightmares and scared to go outside."
A jury at Glasgow Sheriff Court convicted John Cassidy, 26, of assaulting the boy on October 16, 2011 at Fountainwell Road, to his severe injury and the danger of his life. Sentence was deferred.