It’s back to the bachelor pad for Justin Long! The “I’m a Mac” guy was booted from girlfriend Drew Barrymore’s four-bedroom Hollywood Hills home in early November, Star has learned.

And though the two are still dating for now, their relationship could be in danger if he doesn’t clean up. “Drew really had it with being Justin’s babysitter at home,” says a source. “And it’s not just that he made a mess, it’s his attitude. He gets mopey and is a big-time couch potato. She isn’t thrilled about seeing this side of him.”

Problems surfaced almost as soon as the pair — who first sparked in summer 2007, split a year later and reconnected last spring — began living together in August. “Drew hadn’t figured Justin to be a slob, but he started acting like a frat boy and left his stuff all over the place and never straightened up after himself,” says the source. “It drove her nuts!”

Drew finally had enough of the chaotic living situation. But she wasn’t quite ready to call it quits entirely. “She gently told Justin that things would be better between them if he moved back to his own place,” says the source. “Then it would be special when they’re together — they wouldn’t feel like an old married couple.”

The good news? Justin is trying to change his ways. “It was really the wake up call he needed,” says the source. “He knows Drew’s a great girl. He’s going to do everything he can to make her happy.”

He’s 31. She’s 34.