When Grey's Anatomy hunk Jeffrey Dean Morgan hooked up with his Weeds costar
Mary-Louise Parker, he also got a new man in his life: Parker's young son, William.

"We get along like peas in a pod," Morgan, 40, told PEOPLE about the 3-year-old
when he and Parker, 43, attended Thursday's Broadway opening of the new musical
Curtains. (William's father is Parker's ex, Billy Crudup.)

Asked how William was doing, Parker replied, "Fantastic." But on the subject of how
well her two guys get along, Parker – who earlier this month told PEOPLE that her
relationship with Morgan is "going great" – demurred to her date. "You'll have to ask
him for yourself," she said.

"He's wonderful," replied Morgan. "He's great. We spend a lot of time reading,
actually. And I just got back from the park [with him] before we came here."

Being around a child, Morgan admits, "It's a whole new world, a different world,
but it's fantastic. It's great!"