According to mum Kourtney, Mason has been dealing with a case of the green-eyed monster since his new cousin North was born in June.
Following the arrival of her first child, 32-year-old Kim has understandably been pre-occupied with her duties as a mother. But Mason can't work out why his aunt isn't hanging out with him as much as she used to.
Kourtney revealed to People magazine: "He gets a little jealous because he and Kim were so close. He doesn't understand because he's not the big centre of attention."
And the three-year-old's now distant behaviour has left Kim feeling confused too.
Apparently, Kim doesn't know what to say to her nephew to reassure him and instead, she feels that the change in their relationship is due to him getting older and growing up.
To ensure that Mason doesn't feel neglected, 34-year-old Kourtney admitted that she has been piling on the affection, as well as other members of the Kardashian clan.
However, Mason's 14-month-old sister Penelope can't get enough of little cousin North and gets "excited" every time Kim arrives with her.
Kim and boyfriend Kanye West welcomed their first child into the world on 15 June in Los Angeles and Kim has been keeping a low-profile ever since, while she focuses on being a mother out of the spotlight - and dyes her hair blonde!
Meanwhile, older sister Kourtney and longtime boyfriend Scott Disick are often seen out and about with their tiny tots.
Most recently the mother-of-two was spotted at the new Thomas The Tank Engine film premiere with Mason and the pair even posed for an adorable photo with fake moustaches and a train driver hat. Aww!
Check out our gallery of little Mason's most stylish outfits below.