Ashlee Simpson Ross Celebrates Son Bronx's 15th Birthday: 'Thank You for Making Me a Mom'

Ashlee Simpson Ross' son is growing up.

On Monday, the singer, 39, shared a photo of her son Bronx Mowgli to her Instagram Story, taking a minute to celebrate the teenager as he turned 15.

Simpson Ross posed for a selfie with her son, who looked up from his phone and popcorn to take a picture with his mom, while in a movie theater.

"Happy birthday BRONX," the proud mom wrote across her photo. "Thank you for making me a mom! I can't believe your [sic] 15."

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Simpson Ross shares Bronx with ex Pete Wentz. She is also mom to daughter Jagger, 8, and son Ziggy, 3, with husband Evan Ross, 35.

At Jagger's birthday in August, the mom of three shared a family photo to her Instagram which included a rare peek at Bronx.

This past year, Simpson Ross chatted with Jennifer Hudson on her eponymous talk show about Bronx, sharing that he'd been growing up so quickly. The "Pieces of Me" singer marveled at how her oldest is "so tall."

"He's like 5' 9½", he towers over me," said Simpson Ross. "He's really chill, though, so I'm lucky for that." She also noted that Bronx's "voice is deep, he's a young man."

She also opened up about a recent mishap that happened when she went out to dinner with her son and husband. "I'm sitting next to Bronx and Evan is on [the other] side, and we're at one of those movies where they offer you wine or something, and they were like, 'Oh sir [to Bronx], would you like something?'"

Simpson Ross quickly moved to correct the waiter, saying, "He's not my friend — well he's my friend, but this is my kid, my 14-year-old son."