AceShowbiz -

Scott Disick is dating Sofia Richie after his split from baby mama Kourtney Kardashian. But just because he's in a relationship with the 19-year-old model, it doesn't mean he loves the fact that his ex-wife has "become so serious" with her boyfriend Younes Bendjima.

"Scott is still insanely jealous of Younes," an insider tells E! News. "He just can't come to terms with the fact that Kourt has found true happiness with someone else." And even though Scott is "happy" with Sofia, he "can't deal with someone else having Kourtney."

"It makes things between them tense and unpredictable," the source continues. "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."

Kourt and Younes started dating last year after meeting in Paris. And since then, the "Keeping Up with the Kardashians" star is "very happy" and "in love" with Younes. "Scott is hurt she chose someone else and it really bothers him. He can't control his emotions about it and lets it all out," the source adds.

It seems nothing has really changed for Scott in regards to Kourt's beau. In an episode of "KUWTK", Kourt revealed that her ex, with whom she shares son Mason and daughter Penelope, flipped when he found that Younes is "24 years old."

Speaking with her sister Khloe Kardashian, Kourt said that "Scott called me at like two in the morning," adding, that he "never calls me late at night, ever." She went on telling Khloe, "He was just ranting, like, 'What are we doing in life? What are you doing? What am I doing?' And I'll go, 'But what are you doing?' And then he'll say, 'But what are you doing? Is it any better? What are you doing?' "

Kourt said that "by the end of the conversation, I was just like, 'I have a boyfriend, and he's 24 years old! It's not the craziest thing that's ever happened in the world!' " She told Khloe that Scott "was like, 'He's your boyfriend?' And then he hung up the phone." Khloe told Kourt that using the word "boyfriend" was definitely a "trigger" for her ex. When asked if she called him back, Kourt replied, "No, I just went to bed."