The 'Cougar Town' star made the revealation on Ellen DeGeneres Show.
"They're going to have a baby boy. I hope I'm allowed to say that," Cox said.
Cox and Arquette's 9-year-old daughter, Coco, is excited to be a big sister.
"Yeah, she's really excited. At first she was nervous, because, you know, it's a big deal. At first, I was nervous, I was like whoa, heavy. Now it's great and we actually all had dinner the other night, the whole family," the 49-year-old actress said.
Arquette and McLarty have been together for almost two years now following his split from Cox in 2010.