50 Cent Asks Court to Stop Rick Ross' Ex Tia Kemp From Posting About His Son

50 Cent is asking a judge to step in and stop Rick Ross' baby mama, Tia Kemp, from allegedly cyberstalking his 14-year-old son, Sire ... TMZ has learned.

According to court docs obtained by TMZ, 50 claims Tia has repeatedly targeted his son online, making nasty comments about the teen ... including calling him "slow" and claiming he's on the autism spectrum.

50 also takes issue with Tia allegedly posting a photo of Sire alongside a sexually explicit image of a woman the rapper's filing suggests is his mother, Daphne Joy.

According to the filing, Tia allegedly targeted Sire in four separate online publications over just six days, including an October 25 interview in which she again called the teen "slow" and claimed he "can't talk."

50 claims his attorneys previously sent Tia a cease-and-desist letter demanding she remove the offending posts and stop publishing the material ... but she allegedly fired back by posting part of the letter with the caption, "F u and ya lawyers."

The rapper argues Tia's behavior has caused his son humiliation, embarrassment and mental anguish, and believes the alleged harassment will continue unless the court intervenes.

50 is asking a judge to issue an immediate temporary injunction prohibiting Tia from stalking, cyberstalking or harassing Sire, followed by a final injunction after a hearing.

The petition was filed Monday in Miami-Dade County, Florida, on behalf of Sire, who is identified by his initials in the court documents.