Officers said they were called to the zoo at 13:34 BST on Thursday.
Police said the boy, who was from Cambridgeshire and visiting the zoo with his family, sustained serious injuries “while in the enclosure” and was pulled out by zoo staff.
He received medical treatment at the scene before being taken to Addenbrooke’s Hospital in Cambridge.
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”.
“Tracey was the first on the scene, but she was then joined by her husband Andy and their youngest son Edward. All three were involved in the rescue,” he added.
“All three were heroes… they’re a close-knit family and they’re still quite shaken by it.”