Kaleb Bussenschutt of Hillbank, South Australia, near Adelaide, gets violently ill if he eats even a crumb, his mother, Melissa Bussenschutt, tells the The (Adelaide) Advertiser.
All he can take is water and lemonade, she says.
Doctors have "no idea" exactly what is wrong with him, she says.
"If we go out to dinner, to a restaurant, he gets a cup of ice and he'll say, 'What are you eating? What does it taste like?' or 'It's not fair,'" she tells the newspaper.
At his birthday party in February, "he still wanted a birthday cake, but he wanted his sister to eat it," Bussenschutt says.
She adds, "he still wanted to blow out his own candles."
Professor Peter Sly, director of clinical services at the Telethon Institute for Child Health Research in West Perth, Western Australia, tells the newspaper Kaleb's condition is "very rare."
"I've heard of severe absorptive problems that do these sorts of things," he says. "It often unclear whether it's a true allergy or an intolerance."
"The bowel is just so irritable it just can't tolerate normal foods," he says.
The Bussenschutts have organized a bike ride from Melbourne to Adelaide next year to raise money for research into Kaleb's condition, the newspaper says.