"Things became official over the past week while they were in New York City together," the source told Us.
The 34-year-old E! personality and the 19-year-old model were spotted in public together on Friday grabbing coffee and shopping.
"Since returning to L.A., Scott and Sofia are inseparable. They are spending every day together. Sofia seems smitten," a source tells PEOPLE.
In May, the two sparked romance rumors after Lionel Richie's daughter was pictured cuddling Disick aboard a yacht in France.
Since then, the two have traveled to Las Vegas together, were spotted out in Malibu and in New York a few days ago for Fashion Week.
After the two were first seen together, Richie took to Twitter and said they were just friends "Just so everyone can get their panties out of their a–es, Scott and I are just homies," she wrote.
According to PEOPLE's source, the two are hooking up, but it's not serious. "It's obvious that she looks up to Scott. She constantly giggles around him," says the source. "They are very flirty."
"Scott takes care of her, opens the car door and pays for everything," the source adds.
Disick shares kids Mason, 7, Penelope, 5, and Reign, 2, with his ex Kourtney Kardashian.
According to Us Weekly's source, the age difference between Disick and Richie might be an issue.
"It's so weird. Not just the age difference, but mostly the fact that Sofia had been a family friend for years, and was close to Kendall and Kylie," the source said.
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