On Monday, the Younger star, 30, swung by The Late Late Show with James Corden, where she opened up about how her son is prepping to be a big brother.
Duff announced on Friday that she and boyfriend Matthew Koma are expecting their first child together, sharing a sweet photo to Instagram of the two smooching along with a message saying the couple “couldn’t be more excited” about having “made a little princess of our own.”
While the news of Duff’s pregnancy was met with messages of congratulations from her fans, Luca was apparently a little less thrilled.
“The beginning was a little, a little a… he was surprised,” Duff told Corden. “And now he’s warming up to the idea.”
Want all the latest pregnancy and birth announcements, plus celebrity mom blogs? Click here to get those and more in the PEOPLE Babies newsletter.
Luca’s apparently gotten comfortable enough with his sister-to-be that he’s started telling friends at school about his mom’s pregnancy, even if he’s taken a little liberty with the story.
“He told his friend at school the other day that he’s having a sister that’s part red-butt monkey,” Duff joked. “So we’ll see! It’ll be a first. That’ll be really cool.”
He’s also thought of his new sibling’s name.
“We’re up for any kind of suggestions, but Luca thinks ‘Cofant Croissant’ is like, the coolest name,” Duff said, adding that Luca has become pretty insistent on the choice. “There would be no confusion. And when I just refer to it as ‘Cofant,’ and I’m like, ‘That would be so cool, no one at school’s going to be named Cofant,’ He’s like ‘Croissant. Cofant Croissant.’ It goes together. There’s no separating the two.”
“It’s concerning,” she added. “I think he has pretty good taste… It’s pretty strong right?”
News of Duff’s pregnancy comes seven months after Duff confirmed she and Koma were back together. The duo dated for a few months before breaking things off last March, only to rekindle their romance in December.
“It’s going so great,” she said of their relationship on The Talk, admitting that now is actually the third time they had dated. “Timing is such a big deal… third time’s a charm!”
RELATED VIDEO: Hilary Duff Is Pregnant! Actress Expecting ‘a Little Princess’ with Boyfriend Matthew Koma
In September, the Younger star began dropping hints that she and the 31-year-old singer/songwriter were back together by sharing Polaroids of herself strumming a guitar and blowing out the candles on her birthday cake.
“I think that you have history and a past with someone and love, and just because it doesn’t work out the first time, the second time, as long as there’s not too much damage done, then, it can always work out again,” she added on The Talk in December.
RELATED VIDEO: Hilary Duff’s 5-Year-Old Son Luca Blows Kiss While Water Tubing
Meanwhile back in March 2017, around the time of their breakup, Duff opened up to Redbook about the potential of adding to her family — revealing she had “gotten over the fact that I’m not going to have a baby that’s close in age” to Luca.
“It sucked when everyone would be like, ‘When are you having another?’ ” said the actress, whose divorce from former professional hockey player Comrie was finalized in 2016. “I’d be like, ‘Do you know what’s happening right now? I’m not having another.’ “
“I’m cherishing the now,” she continued.
Duff’s baby-to-be will also mean that she and her sister Haylie will now each have two children. The elder Duff welcomed her second daughter, Lulu Gray, on June 5.