JENNIFER LOPEZ’s much-younger beau, CASPER SMART, is terrified that he’s about to get dumped for another handsome young stud, says a source.

Former Calvin Klein model Ryan Guzman, 26, was recently cast as J.Lo’s teenage boyfriend in her upcoming flick, “The Boy Next Door.” And Casper, who’s also 26, fears that the 44-year-old Latin spitfire will trade him in for a new boy toy, the source adds.

“Casper is threatened by Ryan,” disclosed an insider. “Ryan is hot, ambitious and a real charmer. Casper felt like yesterday’s news when Jen­nifer came home giddy and announced that Ryan landed the coveted role opposite her.

“She raved about Ryan’s tal­ent, about what a bright career he has in front of him, and about the sizzling chemistry they generated during Ryan’s audition.”

Meanwhile, Casper’s role in Jen’s life seems to have devolved from ever-present arm candy to glorified ‘manny.’

“He runs her errands, grocery shops, picks her kids up from school and prepares meals at home,” explained the source. “Casper doesn’t think he stands a chance once Jen and Ryan start filming their red-hot love scenes.”

Casper, who met Jennifer when he worked as one of her backup dancers, “knows better than anyone that Jen has a weakness for handsome young men she works with,” the source added.

The ENQUIRER has learned that Ryan, like Casper, has a bit of a sketchy past. He was busted for DUI in April 2011 in his hometown of Sacramento, Calif., while Casper has had multiple brushes with the law.

“Jen has a soft spot for bad boys,” the source continued. “But she’s also traditional and wants a strong man with his own identity. Jen complains that Casper isn’t ambitious. And she’s been bragging that Ryan’s go-getter attitude is a huge turn on. Jen is ready to upgrade her boyfriend – and Ryan looks like a perfect candidate.”