Tristan Thompson was reportedly honest with Khloe Kardashian from the beginning about his paternity scandal.
It's claimed he always told Khloe he would be taking the paternity test, despite being convinced he was not the father.
Khloe and Tristan recently sent a cease and desist letter to Kimberly Alexander, who claims Tristan is the father of her child.
Tristan admitted he had sex with Kimberly, but insists it was years before her son was born.
The child in question is reportedly between the ages of four and five.
Tristan is already father to three-year-old Prince with ex Jordan Craig, and two-year-old True with Khloe.
A source told The Sun : "Tristan has been open with Khloé from the beginning when it comes to the paternity test.
"He told her that he needed to take the test, but also that he didn't believe that he is the dad."
The on-off couple claim a paternity test has already proved Tristan is not the father, but Kimberly disputes the test result as it was from a clinic already used by the family.
Another source told TMZ Tristan slept with Kimberly during his rookie season around 2011-2012, meaning the age of the child would not add up.
Kimberly has claimed on social media that Tristan is "neglecting" his son and has done since he was born.
A lawyer for Khloe and Tristan sent a letter ordering Kimberly to "immediately stop defaming them with malicious defamatory lies and specious fabrications."
They also claimed a paternity test had already proved Tristan was not the father.
"After it was indisputably established by a paternity test performed by one of the most reputable labs that Mr. Thompson is not your child's father, we thought that would be the end of this," they wrote.
"Instead, you have continued to spread outrageous lies about my clients, including ridiculous fictional conspiracy theories that you conjured up claiming that my clients supposedly somehow falsified the paternity test results since members of the Kardashian family had previously used the same trustworthy lab."
They added: "It is obvious that the reason you are spreading these lies about my clients is because you want your 15 minutes of fame. My clients will not tolerate your despicable conduct."
