"He's very loving and sensitive," the reality star, 31, told PEOPLE at the Mercedes Benz Polo Challenge at the Blue Star Jets Field in Bridgehampton, N.Y., over the weekend. "We were laughing with his parents about that. Scott's their little baby boy. And they were joking about [the rumors]. Scott's obviously not a monster."
In fact, Kardashian says, being a dad has just made her appreciate their relationship more.
"[Scott's] doing fantastic [as a father]," she said. "I love watching him with Mason. He makes him crack up so hard. He makes him laugh. It's fun to see them together."
But their son also seems to love spending time with his mother. "Mason is such a mommy's boy," Kardashian said.
For Disick, watching his girlfriend grow into motherhood has also drawn them closer together. "It's just one more quality that I love about her now," he said. "I could never have imagined her to be the amazing mother that she is today. And now it's just one more reason that I love her."
"It's a whole different world now that I have a child," he said. "And, yes, I have grown up quite a bit."