The reality star was spotted partying until 5 a.m. last week in New York City, and now, a source tells PEOPLE he "was a total mess."
"He was doing well for a long time. It's sad and worrisome," the source adds.
Disick, 32, has since returned to Los Angeles and the source says "hopefully he can go back to being healthy" now that he is back.
"He doesn't want to disappoint Kourtney or his kids" the source says.
But a second insider tells PEOPLE that since Disick returned to L.A., he has "been low-key" and seems to be doing much better.
"He knows that he is a mess again and is seeking help," the insider adds.
Kardashian, 36, ended her nine-year relationship with Disick in July after he was photographed cozying up to another woman in Monte Carlo.
Though Disick continued to party for some time after the breakup, he checked into rehab in mid-October and has since completed treatment.
On an appearance on The Ellen DeGeneres Show last month, Kardashian said she and Disick were "getting along" as they co-parent their children and that while Disick will always be family, they aren't back together.
VIDEO: Kourtney Kardashian and Scott Disick Spend Quality Family Time at Home with Kids
Disick then went on Khloé Kardashian's talk show, Kocktails with Khloé, and said many people have the "wrong impression" of him.
"The only thing I'm trying to focus on is calming down and focusing on the kids and trying to be a good dad," he said.
Disick and Kardashian were last photographed together during a friendly shopping trip to a Williams-Sonoma store in Calabasas, California, earlier this month.