Koenig starred as the character Boner alongside Cameron on the long-running television show Growing Pains. The actor, whose father called him "despondent" the last time they spoke, went missing after a February 14 trip to Canada.
Cameron offered his condolences in a statement on Monday.
"I am praying for his family during this time of distress and for his safe return", Cameron said. "Andrew, if you’re reading this, please call me."
Kirk Cameron is an evangelical Christian whose work since Growing Pains has often included Christian-themed productions such as the Left Behind series.