The singer forgot his words and appeared to fall off the stage during his recent performance in Agoura Hills, California, reported People magazine.
Cassidy, who watched his grandfather battle the disease and witnessed his mother "disappear" into dementia until she died at age 89, said of his diagnosis, "I was in denial, but a part of me always knew this was coming."
He added, "I want to focus on what I am, who I am and how I've been, without any distractions. I want to love. I want to enjoy life."