“When friends or yours or family members begin to say to you, ‘Remember? I just told you this two days ago,’ and there’s no memory of it,” Cassidy tells host Dr. Phil McGraw, “that’s when I began to be very concerned.”
The 66-year-old actor also revealed that heartbreaking lesson he shared with his son, Beau, after Cassidy saw his own mother “disappear” to the same disease he would be diagnosed with in 2015.
Cassidy shared with McGraw that he told the 26-year-old: “I want you to promise me you’ll find a way to let me go and don’t let me live like that.”
The actor previously opened up about his personal relationship with Beau, telling PEOPLE: “He’s just one of the best people you’ll ever meet in your life and it’s that that I’m proud of.”
“He knows that I’m just a little off. Sometimes he cocks his head at me a little bit and goes, ‘Dad do you remember?’ and I say, ‘No, I know I do, I remember.’ He’ll give me reminders once in a while,” he added. “He’s such a beautiful person.”
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Cassidy also recently told PEOPLE that he had watched his grandfather battle the disease and witnessed his mother fight against memory loss for decades until she died at age 89.
“In the end, the only way I knew she recognized me is with one single tear that would drop from her eye every time I walked into the room,” he said. “I feared I would end up that way.”
Cassidy is set to appear on Dr. Phil Wednesday (check local listings).