Scott Disick can't seem to put the booze down even though him and Kourtney Kardashian just welcomed their second child Penelope Scotland.

Their relationship is in crisis. Disick is "on a downward spiral" and "more of a mess than I've ever seen him," an insider source told RadarOnline.

Sister Khloe Kadashian "stepped in" and confronted Kourtney about Disick's drinking habits, said the source.

Even though the couple should be ecstatic over their six-week-old, the pair is "the worst they've ever been."

"Scott is never alone with Penelope because he is so irresponsible," said the insider, adding Kourtney even encourages him to leave the house so she doesn't have to deal with him. "Kourtney doesn't even like for him to hold [the baby] because she's afraid he'll drop her."

And Kourtney had good reason to believe Disick can't be trusted with the kids.

On Aug. 10, Disick left their 2-year-old son Mason in his Mercedes SUV with the windows up - on a day when temperatures reached as high as 107 degrees in Calabasas, Calif. - while he was browsing magazine at a newsstand, an observed told the National Enquirer.

"He was out of the car for at least five minutes and should have never left his son alone!" the source told the Enquirer, according to RadarOnline. "The car was parked in the hot sun with the boy alone in the back. The car was not running, so the air conditioner wasn't on, and the windows were rolled up."

But why is Disick back to his partying ways? The answer is simple, according to a RadarOnline source: boredom. "He's a 29-year-old dad with no real job and nothing to do. So he's partying again and drinking heavily."

The Kardashians are "rallying around Kourtney, [as] Scott isn't being a good role model or dad."

Mother Kris Jenner is "furious that Scott's out in public acting like a drunk," an insider said. "She'd rather kick him out and be done with him until he grows up.

"The animosity behind them is really bad."