Well, that was fast. Looks like Jesse McCartney has already gotten bored with Hayden Panettiere and is hooking up with his ex-gf Katie Cassidy. A source close to the pop star tells E! that Jesse has ditched Hayden to get back together with the Melrose Place star…

Cassidy, 23, and McCartney, 22, met when they were both 15 while he was touring with the group Dream Street. The two dated from 2004 to 2007: “They have both grown up a lot and haven’t had much luck dating other people,” the mutual friend tells E! “Since they feel so comfortable with one another, they decided to give it another shot.”

The source adds that McCartney’s camp is psyched about the news: “Katie is on a strict schedule with Melrose Place and Jesse’s people really want him to stay out of clubs and live more of an adult lifestyle,” says our insider. “They think Katie will keep him on track.”

So will Cassidy have some competition from Jesse’s other female suitors? “Katie was less than thrilled when she heard the news that her man was partying with Hayden Panettiere at LA hotspot, Wonderland,” ‘fesses the friend. “Katie isn’t the jealous type, but she knows Hayden has had the hots for Jesse for years now.”

Cassidy’s rep tells E! the girl-on-girl drama is premature: “Katie and Jesse are the greatest of friends, but there is no truth to them being back together romantically.”