Arnold and Klein, a.k.a. TLC's The Little Couple, adopted the boy (who, like his parents, has a form of dwarfism) from China and have marveled at his sweet personality.
William Rijin Klein is "a really happy guy and has transitioned better than any of us expected," Arnold told the New York Post Tuesday. Added Bill, "He's got a way with people."
On the new season of The Little Couple, Klein talked about feeling a connection with her son before they even met in person.
"The moment I saw him, I knew that I wanted [him]," she said. "I could just envision myself taking him home and having him running around the living room and him being part of our family."
Arnold and Klein struggled for years to have a child of their own before deciding upon adoption.
The new season of their show premieres April 30 at 10 p.m. ET on TLC.