The Very Cavallari star, who is currently going through a divorce with the former NFL athlete, took to Instagram on Thursday to pay tribute to their son on his special day.
"Happy 6th birthday my little man. Jaxy, you have such a sweet, sensitive side yet have the ability to make everyone laugh with your incredible sense of humor and wit," Kristin wrote in a message to her son. "You light up a room and make this world a better place. I'm so thankful to be your momma. Love you forever angel."
Kristin and Jay are also parents to son Camden Cutler, 7, and daughter Saylor Cutler, 4. The former couple shocked their fans in late April when they announced their split after 10 years together. Kristin and Jay both announced their plans to divorce in a message on social media on April 26.