The notorious party animal is on a "downward spiral", reports Radar Online.
A source told the entertainment site, "Scott isn’t being a good role model or dad,' adding, "he's more of a mess than ever."
Despite welcoming baby Penelope Scotland — sister to two-year-old Mason — in July, Scott and Kourtney are apparently "the worst they've ever been".
So bad, in fact, that Kourtney can't even trust her baby daddy to look after the newborn.
"Scott is never alone with Penelope because he is so irresponsible. Kourtney doesn't even like for him to hold [the baby] because she's afraid he'll drop her," said the source.
The unemployed father-of-two is said to be hitting the booze in an effort to combat boredom.
"He's a 29-year-old dad with no real job and nothing to do. So he's partying again and drinking heavily," said another source.
News of the couple's relationship crisis comes after Scott reportedly left son Mason in a car in 41 degree heat on August 10.
An eyewitness told The Enquirer the boy spent a good five minutes in the back of his dad's Mercedes SUV while Scott browsed magazines at a nearby newsstand.
"The car was not running, so the air conditioner wasn't on, and the windows were rolled up," said the witness.
Not surprisingly, Kourtney was mortified.
"All she could think about is that Mason could have died," said the witness.
It's certainly not how you'd want a new dad to behave but Scott's stuff-ups might finally get mum Kris Jenner off Kourtney's back about marrying her bad boy beau.
"[She] is furious that Scott's out in public acting like a drunk," an insider told Radar.
"She'd rather kick him out and be done with him until he grows up."