“David Cassidy once had a bigger fan club than Elvis & the Beatles. If you’re a fan, now is a great time to send him your love & best wishes,” Bonaduce, 57, tweeted on Monday evening.
He and Cassidy, 66, played siblings for nearly four years on The Partridge Family from 1970 – 1974.
Bonaduce, who portrayed the middle child in a family of five children, was the first former Partridge costar to speak out following Cassidy’s announcement.
“I was in denial, but a part of me always knew this was coming,” Cassidy told PEOPLE about fighting the memory loss disease.
The actor responded Tuesday on Twitter by sending his own message of thanks to fans: “Thank you to everyone who has reached out offering their love and support.”
The realities of dementia hit very close to home for Cassidy, who watched his grandfather battle the disease and witnessed his mother “disappear” into dementia until she died at age 89. As part of coming to terms with his diagnosis, he made the decision to stop touring as a musician to concentrate on his health and happiness.
“I want to focus on what I am, who I am and how I’ve been without any distractions,” he said. “I want to love. I want to enjoy life.”