EXCLUSIVE. Tommy Lee’s son lost control and barged into Tommy’s bedroom, challenged him to fight, and then knocked him out cold – so violently his fiancee thought he was dead … TMZ has learned.

At this point, our Tommy sources say Brandon burst into the room, yelling at his dad, and challenged him to a fight, but Tommy refused and told him to get out.

Our sources say Tommy was unconscious for several minutes and had blood coming out of his mouth, and his fiancee actually thought he was dead. We’re told after the knockout, Brandon pulled out his phone and began filming before Brittany called the cops.

As we reported … a source connected to Brandon claims Tommy was drunk, but we’re told he only had a glass of wine at dinner.

Brandon says Tommy came after him aggressively and he punched him in self-defense.

We’re told the medics who took Tommy said he suffered a concussion.

Tommy filed a police report against Brandon and cops are investigating.