The 27-year-old NBA star dedicated a sweet post to Prince, his son with Jordan Craig, on the toddler's birthday Wednesday.
Thompson, who plays for the Cleveland Cavaliers, shared a cute photo of Prince holding a basketball. A second picture shows the toddler smiling while bucked up in a carseat.
"Happy birthday to my son Prince, so blessed the man upstairs chose me to be your pops. You're my motivation everyday. Daddy loves so you much!!!" Thompson wrote, adding a crown and heart emojis. "#MyTwin."
Us Weekly said Thompson had a low-key celebration planned with his girlfriend, "Keeping up with the Kardashians" star Khloe Kardashian.
"Tristan has a game tonight at home, so they don't have anything big planned, but they will do dinner after to celebrate," a source said.
Thompson is also parent to 8-month-old daughter True with Kardashian, who gave birth in April following reports Thompson cheated during her pregnancy.
Kardashian explained her decision in November to have Thompson in the delivery room.
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"I made it very clear that I was still disgusted by his actions but I wasn't going to let ANYTHING ruin this moment I've been waiting for my ENTIRE life," the star tweeted.
"Regardless of what Tristan did to me, I was never going to take that moment away from True or Tristan," she said.
"He is her father and they both deserve to love one another as deeply as they possibly can."
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