The 34-year-old reality star is now in a relationship with Sofia Richie, but according to sources, that hasn't helped ease his jealousy surrounding his ex-girlfriend Kourtney - with whom he has Mason, eight, Penelope, five, and Reign, three - and her handsome new man.
A source said: ''Scott is still insanely jealous of Younes. He just can't come to terms with the fact that Kourtney has found true happiness with someone else.''
The insider insists Scott - who dated 38-year-old Kourtney on and off from 2006 to 2015 - is ''happy'' in his romance with 19-year-old Sofia, but ''can't deal with someone else having Kourtney.''
And whilst the pair are trying to act civil toward one another for the sake of co-parenting their brood, Scott's rumoured jealousy has made things ''tense'' between them.
The source added: ''It makes things between them tense and unpredictable. 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.''
'Keeping Up with the Kardashians' star Kourtney began dating 24-year-old Younes in 2016, and is said to be ''very happy'' and ''in love'' with the hunky model.
The insider told E! News: ''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.''
Younes is Kourtney's first serious boyfriend since splitting with Scott, and last year it was reported Scott ''couldn't stand'' seeing pictures of her with the former boxer.
A source said at the time: ''Scott doesn't like Kourtney dating Younes at all. He doesn't want to see photos of them or know about it. 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.''