"We are at 12 weeks now, so near the end of the first trimester," the 38-year-old son of Caitlyn Jenner told the outlet. "We're madly in love and we are very excited about this!"
The shocking pregnancy news comes less than a year after Brandon and Leah announced that they were divorcing. The couple released a joint statement about their split in September 2018.
"It is with love in our hearts that we feel it's time to share some personal news with you all," wrote Brandon and Leah in a statement. "After celebrating fourteen beautiful years together, we have lovingly come to the decision to end the romantic aspect of our relationship."
Leah is the daughter of the Eagles' bandmate Don Felder.
As Radar readers know, Brandon and Leah, both musicians, garnered their own fan base after making guest appearances on Keeping Up With The Kardashians.
The former married couple share one daughter, Eva, 4.
Brandon and Cayley have already shared the baby news with Eva, the couple told PEOPLE.
"We told her together," Brandon shared. "Leah, Cayley and I sat down with her and told her what was happening, and she was very excited and happy. She's super excited to be a big sister."
"Leah and I split up in a really beautiful way, and I think we handled it in the best way possible," he said. "It was a process, and it took some time to finalize."
Brody Jenner's brother added that he is "so happy" to start a family with Stoker.
In recent months, Brandon has posted about his new love. The expectant parents shared photos from a Hawaii getaway on July 4.
However, sources have previously told Radar that Brandon's relationship with dad Caitlyn is fractured. An insider close to the Jenners told Radar that Caitlyn was "so wrapped up" in her relationship with Sophia Hutchins that she didn't even call Brandon after his divorce news broke.
Leah, who now goes by Leah James, has yet to break her silence on the news.