The boy, Mateo Bravery Bratt, was born in Los Angeles and weighed in at 7 lbs. 7 oz. Both mother and son are doing well, the spokesman says.
The pair already have a daughter, Sophia, who is 2 1/2.
Cast as lovers in 2001's Pinero, Bratt, 41 and Soto, 38, began their romance when they met up for the movie's premiere at the Montreal World Film Festival.
"It started as a friendship, and little by little, they found they had something in common," Soto's friend Nicky Corello told PEOPLE in 2002.
Soto – a former model from Brooklyn who kicked off her movie career as a Bond girl in 1989's Licence to Kill – was married to actor Costas Mandylor for three years.
Bratt recently told PEOPLE that fitting in long hours on his new show and time with his family is tough – but he loves it.
"That (work-life) balance needs constant attention," he said. "I'm hurting for sleep, but it's all worth it."