The glamour model was 'frantic' after the eight-year-old fell ill at home and had to be admitted to the East Surrey Hospital in Redhill.
She fears that severely autistic Harvey might develop complications from the illness, which is especially dangerous for children under the age of 14.
Harvey's former stepfather Andre raced to join his ex-wife at the boy's side after cancelling a planned visit to turn on the Christmas lights at a London store.
It was the first time the pair have been alone together since their acrimonious break-up, apart from a few terse exchanges as they handed over their children.
Harvey was taken to hospital by Katie's mother Amy and a nanny, as the mother-of-three was away from home on her book tour.
Doctors told Katie that Harvey will need to remain in hospital for at least 48 hours while they assess him.
A source told the Mirror: 'Harvey had a massive temperature and couldn't keep any food down either.
'More worryingly, his breathing was also a huge concern and there was no option but to urgently seek medical advice.
'He's a very poorly young man.'
Her spokesman said: 'Harvey woke up on Wednesday with a very high temperature and flu-like symptoms so he was taken to hospital, as he always would be with such symptoms because of his disabilities.
'Doctors decided to keep him in hospital overnight for observation as a precautionary measure because of his underlying health problems.
'Katie was in touch with Peter from the moment it was decided to take Harvey to hospital and Peter visited Harvey in hospital.
'Katie also informed Harvey's biological father Dwight Yorke of the situation.'
Katie's mother will look after Katie and Peter's children Junior, four and Princess Tiaamii, two, while their parents remain at Harvey's bedside.
I love these two. I hope they get back together, they're like a British version of Coco and Ice-T and they're so tacky it's fabulous.