Chris Spencer visited Concord Park in Sheffield on November 17 with his son Reaf, aged six, but things soon took a turn for the worse after the youngster asked to stroke a dog.
Chris said the dog jumped on his son Reaf and bit him despite the owner claiming that it was friendly with other people.
The 33-year-old from Chesterfield now wants to raise awareness about the incident to stop other children from being attacked.
Chris said: “We visited Concord Park in Sheffield as we do every week. My son saw a dog, and I said to the lady, is he friendly, and is there any chance my son can stroke him.
"She said he’s beautiful with people, but he hates other dogs, and as soon as she said the word dog, it growled, jumped up and bit him in the face that quick I couldn’t even stop it.
“My son span around because the dog was still holding onto him, and it has ripped his face. He’s got a deep wound and then two big scratches, but they are still deep enough to be cuts.
“My son has grown up knowing that he can’t just touch other people’s dogs without asking. One, he’s got to ask the owner if the dog is friendly, and two, he’s got to ask if he can stroke it.
"But it just goes to show that even if someone says their dog is friendly, you can never trust that they actually are.”
Reaf was taken to Sheffield Children’s Hospital and treated but later returned after the wound became infected.
The incident has been reported to South Yorkshire Police, who are now investigating.