A rep for the D.A.'s office tells TMZ, charges won't be filed due to insufficient evidence.

TMZ broke the story, Madsen was arrested at his Malibu home on Friday on suspicion of felony child endangerment after allegedly getting into a physical fight with his juvenile son ... who he found smoking pot in the house.

Cops responded to a call about a domestic dispute at Madsen's home -- and when they arrived, we're told they observed signs of injury on the boy. According to cops, Madsen also appeared to be under the influence of alcohol at the time of his arrest.

His son did not require medical attention.