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Tre Mason's mother has told police ... her son should NOT be playing football anymore -- because the head injuries he suffered in the NFL have had a devastating impact on his mental health.

TMZ Sports has obtained police footage from a July 27th incident when cops cited the L.A. Rams running back for illegally riding his ATV around his neighborhood in Palm Beach, Florida.

When they got to his home, cops told his family they were concerned Mason was too busy "f***ing around" and "should be playing football."

That's when Tre's mother dropped some real talk on the cops.

"No, he actually shouldn't," she said ..."because this is the [side of] football that nobody knows about."

"He's 22 in a 10-year-old's mindset right now ... Tre's not himself at all."

Tre's mother says the family could see an obvious change in Tre's personality after the 2015 NFL season ended.

As we previously reported, Tre has been at the center of several police incidents in the past few months -- and his family clearly believes football is the root of the problem.