CAMBRIDGE A 13-year-old boy is being hailed as a hero by his parents for alerting them to a fire in their Cambridge home on Monday. The fire destroyed a multi-unit building, leaving five families homeless. Damage is estimated at nearly $1 million, but residents said it could have been much worse. We ran outside and watched everything burn down, said Nathan Viveiros. It was too quick to comprehend anything. I opened the door, got shot with the flames. Viveiros lives on the top floor of the building. He said his son alerted him about the fire. My whole room was full of smoke, his son said.