The boy's two brothers had to pry the dog's mouth open to make it let go, resulting in a small wound to one of the brothers.
The man said in retrospect he should have done something about the dog earlier, because it had bitten people before.
Whangarei District Council spokeswoman Ann Midson said dog-control officers were called by police to a property in Fairway Drive at 2.30pm yesterday.
"When we got there we discovered that an English bull terrier had bitten the top of the child's leg," she said.
"The dog's owners immediately surrendered the dog to us and asked for it to be euthanised, and a vet will be taking care of that."
Ms Midson said the dog was registered and micro-chipped, and had no history with animal-control officers.
The council's animal-control contractor, Environmental Northland, had said the case was "a very sad situation which arises from time to time," Ms Midson said.
"It's not unknown for a well-known family pet to get possessive when young children come into the home.
"They can take children as a threat."