When can a birth father contest an adoption?

For 2 years my wife and I have been fostering twin boys, age 3 through the Clark County Child Protective Services here in Las Vegas and have always planned on adopting them. The boys’ biological father has not been around the entire time the twins have been with us. We were scheduled to finalize the adoption next month and the father has reappeared and is saying he wants his kids back. He will not consent to the adoption. What chance does he have to fight the adoption if he’s never been a father to the boys, and we are the only family they’ve ever known? The social worker is telling us we may now have to go to a trial to terminate his parental rights. Do we need our own lawyer for that trial?