"He is in the hospital and it's not looking good", a source told E!
Other media sites have reported Cassidy is suffering from organ failure.
After checking into a hospital earlier this week, Cassidy went into a medically induced coma, but he has since regained consciousness, Geffen said by phone.More news: George Clooney Will Star in and Direct 'Catch 22' Series
David's family has been arriving at the hospital to comfort him. Cassidy's representative tells People he needs a liver transplant.
In February, Cassidy announced he has dementia, something both his mother and grandfather battled. Cassidy's health has been in decline for more than two months, and his family has been told to prepare for the worst.
In recent decades, the teen idol was flooded with personal problems that ranged from substance abuse, bankruptcy, divorce, foreclosure and drunk driving violations. He is the stepson of actress and fellow Partridge Family star Shirley Jones.