The late former Nakuru Mayor Joseck Thuo's son wants the will subjected to forensic examination to establish its authenticity.
Nixon Thuo told Justice Heston Nyaga that he suspects his father's signature was forged in a bid to manipulate the will.
He is seeking orders to have the four copies of the will subjected to forensic examination.
The will is key to the distribution of the Sh1 billion estate, and has caused a major rift among the beneficiaries.
Nixon, her mother Susan Wanjiru, who was Thuo's second wife and his sister Mukami, have contested the authenticity of the will.
"We urge the court to subject the four copies of the will to forensic examination to ascertain if everything tallies, we need to know the truth," Nixon urged the court.
The businessman cum politician died on December 27, 2021.
Nixon's siblings James, Geoffrey and Erick insist the will is genuine and the court should rely on its content.
On Wednesday, Nixon told Justice Nyaga that the signatures in the document do not match.
He claimed that one of the copies had an endorsement while the other doesn't and objected to page two to page five of the will.
"The six pages will have different signatures full of mistakes in terms of names, properties he never owned, my name and that of his wives were misspelled," Nixon claimed.
He accused his step-brother James of allegedly forging their father's signatures twice, claiming that Thuo had confided in him.
Nixon stated that he knew his father's signature but what was on the will didn't tally with only the last page being genuine.
"I'm not implicating him because he is my stepbrother, my father had told me that he forged his signature on two occasions in the past," he said.
Nixon alleged that days after his father's death, his driver gave him a copy of the will which was later scanned by his brother Erick and shared to their emails.
At the time, he claimed that the will was one copy but later his brother Geoffrey came with two copies.
To clear the doubts, he said, they took the alleged original copy to his lawyer who observed that it had irregularities forcing him to take his copy to DCI for investigations.
The late Thuo's lawyer John Kagucia, who drafted the document, revealed that drafting the will started in 2014 and was completed on February 3, 2016.
