Sources close to Tommy tell us ... Brandon went nuts after his dad posted a few pictures of his mom, Pamela Anderson, making funny faces, because he felt it disrespected her. We're told he texted his dad -- who was in bed with his fiancee, Brittany Furlan -- from another room telling him to take the pics down.

We're told Tommy shot back at Brandon, asking him why's he's upset by this but not upset that Pam accused him of abuse.

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. We're told when he got up and went to the door to make Brandon leave ... Brandon sucker punched him, knocking him out cold.

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.