Michael Girgenti said that he had a fling with Kourtney in March, 2009, when she was on a break from her partner Scott Disick.
Girgenti has written a letter to the 34-year-old demanding that her three-year-old son should undergo a DNA test and has even threatened to sue her, reported Daily Mirror.
"This is Kourtney's last chance. My next step will be to take her to court and sue her for a paternity test. I'm tired of not knowing if Mason is my child, and I want the constant wondering to end," his letter said.