Nicolas Cage's son pled not guilty Wednesday to a charge of assault with a deadly weapon, stemming from his alleged beatdown of his own mother -- a startling allegation that surfaced back in May ... when TMZ first broke the story.

We got Weston on his way out of a downtown Los Angeles courthouse on the heels of his not guilty plea -- and our photog asked him about what Christina had told us on her way into court ... namely, that she was hoping Weston would get the help she thinks he needs.

Weston snapped right back ... "She needs help." However, he didn't elaborate any further.

As for his mother -- Christina was emotional on her way into the building ... and she was in no mood to talk about their fractured relationship, other than to say Weston should seek help.

Clearly, Christina's healed from the injuries she claims she suffered at her son's hand.

Remember ... Weston was arrested earlier this month after his mother alleged he "brutally assaulted" her back in April during what she characterized as a "manic rage" -- but obviously, Weston is ready to fight this claim ... something his attorney made crystal clear today.

Photos surfaced after the fact that appeared to show Christina with horrific bruises covering one side of her face.