It was a mad scramble to get ready, parenting classes, crib, diapers, and blankets. Then just days before Christmas, they were told the baby would be transferred to their care.
"Our paternal instincts took over and it became a natural thing of how to take care of him," Mercurio said.
They took their son home on a snowy day, riding the same C train where they found him.
As blessed as they felt, the couple knew there would be challenges.
When they first held the boy -- whom they soon renamed Kevin -- at the foster home they found him guarded.
"In fact when we saw him he didn't blink. His eyes were just wide open and his arms were very stiff and tightly crossed across his chest," said Mercurio.
"So we got him in this condition and we thought we just need to love this kid immediately," added Stewart. "So we played with him and build up his trust in parents. Build up his trust in adults -- that he could be cared for, nurtured, and loved. So we showered him with love and touch. Didn't take long. He loosened up."
CNN is not identifying Kevin by his last name or his picture, to protect his privacy. But he knows the story of his discovery.
A quiet family life
Mercurio and Stewart created an illustrated child's storybook, dramatizing the events-- from the subway to meeting his new family.
"One day he asked me: Dad is the story about me?" said Stewart.
"I was very happy," said Kevin.
He likes sports, his school, and his friends.
The family went back to the dark underground station. They were all a little nervous about how Kevin would react.
"I think that was important for him to see and know that because now he has a connection," said Stewart. "I mean it's not just something abstract. He really has seen, and knows, and understands. And he has taking a lot of pride in that spot. That's his station. That's his place. This is where we became a family."
Stewart and Mercurio are not activists. They live, quiet lives in Manhattan and like all parents, find joy and occasional frustration in raising a soon-to-be teenager.
"You know sometimes in life you have to say yes," Mercurio said. "And we said 'yes' to becoming this baby's parents and it was the best 'yes' decision we have ever made in our lives."
Stewart said the story speaks to a core of humanity.
"I mean, deep down when you strip away all those layers, all those labels, we're all human beings and were all connected by certain things that we need in our lives -- love," he said.