"To see her in the role as mother is, like, one of the most amazing things I've ever seen," the motorcycle guru tells ABC's Nightline in an interview airing Tuesday. "She is like a fish to water. ... As soon as she put that baby into her hands, it was automatic and amazing."
He adds: "I found a new trust for her, like I knew she wasn't going to leave, and our relationship blossomed."
James, 41, and Bullock, 45, found out over Christmas that the adoption had gone through, and welcomed Louis, born in New Orleans, on New Year's Day. In March, James's infidelity scandal erupted.
James said seeing the PEOPLE cover in April of his estranged wife holding up their son was a painful reminder that he was suddenly absent from the child's life. "It made me sad, but I realize why things have to be done in a certain way," he says, pointing out that adoption agencies could be critical of him as a father in light of recent events.
Asked why they adopted a black child, James says he "didn't care what color" the baby was. "My only prerequisite for adopting a baby: I want the baby that needs us the most."
Bullock had already been a stepmom to James's three children – daughters Chandler and Sunny and and son Jesse Jr.