According to a statement released Thursday, the couple's second son, Aiden, was born on Wednesday. He weighed 7 pounds, 12 ounces.
"Mother and baby are healthy and blissfully happy," the statement read. No other details were released.
Their older son, Alastair, was born in 2005.
Stewart also has six children from previous relationships, including two children with his first wife Alana Hamilton, one child with Kelly Emberg, and another two kids with former wife Rachel Hunter.
The Associated Press contributed to this report.