Police said the boy, who was from Cambridgeshire and visiting the zoo with his family, sustained serious injuries “while in the enclosure”.
He is believed to have been attacked by at least one crocodile.
He received medical treatment at the scene before being taken to Addenbrooke’s Hospital in Cambridge.
On Monday, police confirmed he was no longer critically ill, but was “in a stable condition in hospital.”
Chris Newman, the director of the National Centre for Reptile Welfare, told the BBC that he had spoken to the Johnson family, stating they were “all very distraught”.
He said Tracey Johnson, husband Andy and their youngest son were all involved in the rescue.
“All three were heroes… they’re a close-knit family and they’re still quite shaken by it.”