Bill Cosby is reacting to the sudden death of “The Cosby Show” alum Malcolm-Jamal Warner … and it’s taking him back to a dark time in his life — his son’s murder.

Cosby’s rep, Andrew Wyatt, tells TMZ … when Bill got the call that Warner died on vacation in Costa Rica, it reminded him of being in the studio filming “The Cosby Show” in January 1997, when he got a call informing him that his son Ennis Cosby was shot dead in a failed robbery attempt.

Wyatt says news of Malcolm-Jamal’s death hit him the same way as Ennis’ murder … the two used to play together when they were younger and “were amazing together.”

We’re told when Bill last talked to Warner, he was excited about a big concert in Minneapolis and felt like he was changing the world.

Wyatt says Bill’s also remembering the time Malcolm-Jamal visited the Cosby family property in Massachusetts and made Bill’s relatives laugh harder than ever.

Bill, we’re told, feels Malcolm-Jamal made a lasting impact with his life and career that will continue to change the world … and he says every young man growing up in the projects back in the day wanted to be like Warner and to have a family like the Huxtables.

TMZ broke the story … Warner died from an accidental drowning Sunday in Costa Rica, where he was on vacation with his daughter. He was only 54.