Choreographer Wade Robson has stepped up his battle with Michael Jackson's heirs by threatening the late superstar's father with legal action. Robson is suing the singer's estate over allegations Jackson abused him as a young boy, but his decision to go public has angered members of the star's family, including his father Joe Jackson, who cast doubt on the choreographer's claim in a video recently posted on TMZ.com. Robson's lawyers have now fired off a cease and desist letter to Joe Jackson, accusing him of slander for the comments he made in the clip. In documents obtained by editors at TMZ, Robson's lawyers write, "Your statement is particularly reprehensible in light of the fact that (Robson) has only recently been able to come forward which is so typically characteristic and tragic for so many survivors of sexual abuse... Any trier of fact would conclude your statement was malicious, intended to further injure our client." The attorneys go on to threaten Joe Jackson with legal action over his remarks. Robson, now 30, took the stand in 2005 during Michael Jackson's child molestation trial in Santa Barbara, California to insist the Thriller hitmaker had never abused him during several sleepovers at his Neverland Ranch and Las Vegas homes.