"He's engaged," according to Torriero's rep, Jack Ketsoyan, who says the pair have been dating for two months.
Late Wednesday night, Stewart, 26, and Torriero, 19, showed up at an XBox 360 party at a private home in Beverly Hills, where Stewart showed off her 5-carat diamond Neil Lane engagement ring to buddies Paris and Nicky Hilton.
A friend of Torriero's tells PEOPLE: "He told me he really likes her and they're having a good time."
The 6'1" California-born Torriero – who excels at surfing, volleyball and football – was a junior in high school when he first shot Laguna Beach: The Real Orange County for the music cable network last year.
In September, he told MTV News: "I'm doing an album, which is very exciting. ... I can't tell you what label I'm with, but it's a really big label push." He also said he was hoping to expand his career opportunities.
"I've been acting since I was young," he said. "Now I get to take what I'm doing and what skills I have and use it to my advantage."
Stewart, who is Rod Stewart's daughter, had been rumored to be replacing Nicole Richie on FOX's The Simple Life after a rift developed between Richie and costar Paris Hilton, but that plan never came to fruition.
Last summer she got into hot water for comments she made in Blender magazine about Jennifer Aniston's looks, calling the former Friends star "homely," and opining: "It's not like she's gorgeous or anything."
After Stewart read that Aniston's feelings were hurt by the remarks, Stewart told TV's Access Hollywood: "It made me feel awful, so I sent her flowers today and an apology."