Luis Brignoni, 41, told the Bradenton (Fla.) Herald his son, Fernando Brignoni, was in the backyard petting the family's Malamute-wolf mix named Sassy through a cage Friday when the dog grabbed onto his hand.
Fernando's brother, Luis, 13, hit the dog to get it to let go, but the dog had bitten off Fernando's pinky finger.
The boys' father said he knew what he had to do.
"I said, 'I've got to go find the finger,' " Brignoni said. "I got my gun and I popped [the dog]."
The elder Luis Brignoni was cutting open the dog to retrieve the finger when a sheriff's deputy arrived at the family's home, the Herald reported Monday.
The finger was sent along with Fernando in a helicopter to St. Petersburg Hospital in Tampa where doctors decided there was too much nerve damage for successful reattachment.
Doctors performed emergency surgery to correct damage done to Fernando's finger, the newspaper said.
"You don't know what you will do for your child," Brignoni said of the incident. "You don't know where you get the strength from."