In 1990 a jury convicted Ohio doctor John Boyle of murdering his 44-year-old wife, Noreen. The couple’s then 12-year-old son, Collier, testified at John’s trial, helping to put him behind bars. For over two decades, Boyle has denied his guilt.

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In an exclusive daytime Dr. Phil airing Wednesday, Collier, now 40, says that when the prosecution told him he didn’t need to testify, he responded, “‘There’s absolutely no way you’re talking me out of testifying, because I know what happened, and this’ – I was very, very adamant about it – ‘This guy’s not going to get away with this.'”

WATCH: Man Who Testified In Dad's Murder Trial At Age 12 Says, 'I Literally Stared Him Down The Whole Time'

In the video above, Collier, who is the subject of the documentary “A Murder in Mansfield” from Investigation Discovery, says he discovered his father was having an affair in the months before his mother was murdered.

Check here to find out where you can watch Wednesday’s episode.

Footage/photos provided by Investigation Discovery. “A Murder in Mansfield" premieres on Investigation Discovery and IDGO at 9 p.m. EST on Saturday, November 17, 2018.

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Subject Of 'A Murder In Mansfield' Documentary Says 'The Purpose Of This Film Is To Show Vulnerability'