The mother of three has been dating model Younes Bendjima, 24, since late 2016.
"Scott is still insanely jealous of Younes," a source told E! News. "He just can't come to terms with the fact that Kourtney has found true happiness with someone else."
The fact that Kourtney and Younes are still together is surprising to most, as the majority of her inner circle figured Younes was a "rebound" relationship when they started out. But, the source said the reality TV star is "very happy" and "in love" with Younes.
"Scott is hurt she chose someone else and it really bothers him," the source says. "He can't control his emotions about it and lets it all out."
Perhaps Scott is being a bit hypocritical since he's moved on and has been dating 19-year-old Sofia Richie. E!'s source said Scott is "happy" with Sofia, but he "can't deal with someone else having Kourtney."
"It makes things between them tense and unpredictable," the source said. "Kourtney never knows when he is going to fly off the handle or go nuts about something. Scott just never thought it would last this long with Younes or become so serious. Nobody really did."
It seems like nothing has really changed for Scott in regards to Kourtney's beau. In May 2017, a source told E! that Scott "doesn't like Kourtney dating Younes at all."
"He doesn't want to see photos of them or know about it," the source claimed at the time. "Kourtney has tried to give Scott a heads up that he might be seeing photos, but Scott can't stand seeing her with someone else, especially Younes."
Another source then added, "Whether they are together or not, Scott gets really jealous when he sees or hears about Kourtney with anyone. He'll always love her, regardless of their status."