The 34-year-old reality TV star has three kids with Kourtney, 38 - Mason, eight, Penelope, five, and Reign, three - but he believes that Sofia's laidback attitude and age makes them more suitable as a couple.
A source said: ''No one expected things with Scott and Sofia to get serious like this, even their close friends are shocked. But they really do work well together.''
Scott and Kourtney had an on/off relationship between 2006 and 2015, and following their split, he admitted he was still in love with the brunette beauty.
But the reality star now looks at the situation in a different light, and is apparently appreciative that Sofia, 19, allows him to ''get away with a lot''.
The insider told HollywoodLife: ''A big part of it is that Sofia doesn't try to control Scott at all. She's one of the most laidback people ever, she's basically Kourtney's total opposite. And her age is another factor, she doesn't put many adult demands on Scott; he can get away with a lot.
''The ironic thing is that by giving Scott such a long leash she's actually got him more in line than he's ever been. Now that there are no rules he feels like he doesn't need to rebel, it's like Sofia's found the secret code or something.''
This comes shortly after Sophia was spotted on her first public outing alongside Scott's children.
The teenage model - who is is the youngest daughter of singer Lionel Richie - was seen heading to a restaurant with her boyfriend and two of his three children.
An insider said at the time that ''everyone was interacting and looking happy during the meal'', adding that Sofia ''seemed comfortable with Scott's kids''.