Whoa! Scott Disick whisked son Mason away after a massive fight with baby mama Kourtney Kardashian, on Monday, a source tells X17online exclusively. The father-son duo hopped a commercial flight (we're told he doesn't have the $$$ for his own private jet, since Sofia Richie's not footing the bill for this one) with his 7-year-old son following a disagreement with Kourt over Thanksgiving plans.

The Kardashian clan is planning a big family get together and after their big blowout, Scott wants no part of it. Our source tells us, "Scott is pissed at Kourtney right now and he knew the way to hurt her the most was to take Mason and do something with him for the holiday. And Sofia Richie is involved in this whole thing, too."

Our source says Sofia wanted Scott to herself for Thanksgiving but the Kardashians still consider Scott part of the family and they wanted him to come along on their family vacay for Turkey Day. The 19-year-old "model," however, isn't thinking in such terms: "Sofia just wants a fun vacation with Scott. Their relationship is kind of built on them going off on 5-star trips together and Thanksgiving was the perfect excuse for a holiday. Sofia doesn't really understand Scott's responsibilities as a parent because she's still a kid herself."

As the Kardashian family prepares for their annual Thanksgiving retreat together, Scott is sending a clear signal that he wants no part of it this year ... and he doesn't want Mason with Kourtney and her family either. He's whisked his boy away for some father-son bonding time and who knows, maybe it'll make the notorious bad boy realize he needs to be present more often!

With the holiday only two days away, the Kardashian fam is waiting to see what Scott will do ... will he keep Mason the whole week or will he cave and meet up with the rest of the group for turkey and fixins? Our source says Scott's not even sure, himself: "Scott just left, impulsively. He's not sure if he'll join up with the family later or not. He wanted to make a point -- to shock everyone -- and it worked. Kourtney's a little worried but she also knows Scott loves his son and wouldn't do anything to put him in danger. It seems like there's part of her that's happy Scott is finally acting like a dad."

Mason looked happy ... finally some attention from dad ...