Twins? Matthew Broderick is "nervous just even thinking about it"

Sarah Jessica Parker Matthew Broderick David Letterman

that he's already nervous.

But the couple's 6-year-old son,

James Wilkie

, can't wait to meet his new sisters.

"He's pretty excited," Broderick, 47, tells David Letterman in an episode of the "Late Show" airing tonight.

"He said, 'We have to get a lot of princess toys.' He's just now thinking, 'Wait a minute, am I (not going to be an only child anymore)?'

Broderick jokes that he's not as confident about raising twin girls. "Oh man, I'm nervous just even thinking about it," he says.

Asks Letterman: "Are you ready for this? Because's not just one child, it's, like, two. It's like one of those Octo-mom deals."

"Yeah, I'm ready. You know, I've asked people, 'What's it like to have twins?' One father said, 'I don't recommend it,'" said Broderick. "But the rest have been very positive."

Broderick and Parker, 44, will welcome the babies later this summer.

Parker's "Sex and the City" co-star,

Mario Cantone

, told People magazine Thursday that SJP has always wanted a daughter.

Parker is "the ultimate girl," Cantone said. "I mean, she's her. She's going to have so much fun bringing the two girls up and teaching them to be young ladies."

| Posted by Lisa Gutierrez, The Star