12:07 PM PT -- We've obtained the declination, and it says Brandon allegedly told Tommy, "Get up old man and fight me" before punching Tommy out cold. The doc says Brandon allegedly then said, "Look at you, motherf*****, you are knocked-out." Tommy's GF then called 911.

Tommy Lee is first and foremost a dad, and that may be why he chose not to cooperate with the criminal investigation targeting his son ... so the L.A. County D.A. has officially rejected the case.

As TMZ reported, Tommy Lee's 21-year-old son, Brandon, coldcocked his dad during an argument last month. Tommy had gone on social media trashing ex-wife Pam Anderson for reliving his domestic violence case during an interview.

Brandon went ballistic, barging into Tommy's bedroom and knocking him out with a sucker punch to the mouth. Brandon told cops he believed Tommy was about to strike him so he punched in self-defense.

Cops launched a battery investigation, but Tommy had a change of heart and wouldn't cooperate. One possible reason ... TMZ got Brandon out after the fight and he talked about how much he loved his dad.