Thomas Ravenel welcomed a son, Jonathan Jackson, with ex-girlfriend Heather Mascoe in June

Thomas Ravenel's family is growing — much to his ex Kathryn Dennis' surprise.

In PEOPLE's exclusive sneak peek at Thursday's episode of Southern Charm, Kathryn, 29, gets a surprising text message about Thomas, 58, while at dinner with friends.

"Oh, Lord have mercy," she says. "Gross, Thomas is such a f---ing liar."

Rattled, she seeks out her longtime friend Danni Baird.

"Thomas got a girl pregnant and she's having the baby. I just found out," Kathryn whispers to her. "It's happening."

Asked how far along the mom-to-be is, Kathryn says, "probably six, seven months."

Kathryn Dennis and Thomas Ravenel
| Credit: Charles Sykes/Bravo/NBCU Photo Bank/Getty Images

Kathryn, who shares daughter Kensie, 6, and son Saint Julien, 4½, with Thomas, says she didn't even hear the information straight from ex — instead, her lawyer broke the news to her.

"My lawyer found out and he confirmed it," she says.

Back at the table, the rest of the group picks up on the shift in Kathryn's mood, so she decides to share the news.

"Thomas is having another baby," she says, shocking everyone. "I'm asking him, 'Why wouldn't you tell me?' And he's like, 'Why would I?' Because [Kensie and Saint] are going to have a sibling, hello!"

"He said, 'Well, half,'" she adds.

As fans know, Thomas welcomed a son, Jonathan Jackson, with ex-girlfriend Heather Mascoe on June 29.

"Meet Jonathan Jackson Ravenel. One 5 weeks old," he tweeted in August, sharing a photo of the newborn.

Jonathan Jackson Ravenel Thomas Ravenel
Thomas Ravenel's son Jonathan
| Credit: Thomas Ravenel/Twitter; Inset: Charles Sykes/Bravo/NBCU Photo Bank/NBCUniversal via Getty

Thomas and Heather, 38, were first linked last April, though they had split by August, a source told PEOPLE. She has two children from a previous relationship.

"We are both very happy," Thomas told theDaily Mail in August. "Heather is a phenomenal mom and they're both doing well."

Kathryn and Thomas' tumultuous romance played out throughout their time together on Southern Charm. The two dated during the first season when she was a 21-year-old college student and split shortly after she gave birth to Kensie. They reunited in 2015, just long enough to conceive their son.

After a lengthy battle, the former couple agreed to share joint legal and physical custody of their two children last year. Ravenel was given "primary legal custody," his lawyer told PEOPLE at the time.

Since then, they have been co-parenting their kids and have spent time together socially.

Southern Charm airs Thursdays (9 p.m. ET) on Bravo.