"Dad, on what would be your 70th birthday, I would want you to know that your incredible spirit lives within us," he wrote. "Our family will be celebrating you and your memory today. We miss you and love you always!"
Zak Williams is the star's only child with his first wife Valerie Velardi. He was also father to Cody Williams, 29, and Zelda Williams, 32, both with his second wife Marsha Garces.
Since his father's death, Zak has dedicated his life to advocating for mental health. He co-founded Prepare Your Mind supplements as well as supporting non-profits and speaking out about his father's struggles with mental health and his own.
"After my dad took his life in August of 2014, my anxiety went into overdrive," he explains on the Prepare Your Mind website. "I found myself depressed, anxious, traumatised and unable to cope with daily life."
He opened up earlier this year in an interview with Oprah Winfrey and Prince Harry on their AppleTV+ series, The Me You Can't See.
"From my end, I didn't get a chance to focus on the private grieving process until about a year-and-a-half after my dad passed away, meaning that I didn't recognise that I needed to grieve privately," he shared.
Zak is now a father himself to two-year-old son McLaurin Clement, and two-month-old daughter Zola June, whom he shares with wife Olivia June.
He has also started to share his father's legacy with his kids.
"I have slowly started showing photos and pictures to my son, things from movies and in animation that my dad was in," he told the Today show last July.
"We have photos and memories around our home and he's drawn to them. And so when he sees the Genie from Aladdin or when he sees a photo of his grandfather, he's drawn to it. And so we talk about it … It's a nice way to carry on my dad's legacy related to our family history."
