The source said Larry Feldman testified Monday in the case in which a 14-year-old boy alleges Jackson molested him when he was 12.
Feldman has counseled the mother of the most recent accuser.
The family in the 1993 case accepted a multimillion-dollar out-of-court settlement from Jackson in a civil suit. The singer was never charged in connection with those allegations.
The source told CNN the latest accuser is now in the Santa Barbara area and is expected to testify before the grand jury soon.
Jamie Masada, the owner of the Laugh Factory who claims to have facilitated the introduction of Jackson to the latest alleged victim, testified before the grand jury Tuesday for about 30 minutes, another source familiar with the case told CNN.
Santa Barbara County District Attorney Tom Sneddon charged the 45-year-old Jackson late last year with seven felony counts of lewd or lascivious acts with a child under 14 and two counts of giving the child an "intoxicating agent."
According to independent.co.uk a California grand jury meeting in secret has heard testimony from the boy who accused Michael Jackson, the self-proclaimed "King of Pop", of sexual assault, according to sources following the proceedings.
Members of the grand jury have been meeting since Monday to hear evidence in the Jackson case. The accuser, who is now 14, was said to have made an appearance on Tuesday. No details of his or anyone else's testimony have been made available. If 12 out of the 19 members of the grand jury vote to indict Mr Jackson, the burden on prosecutors to present evidence in pre-trial hearings and convince a judge that the star should stand trial will be eased. Proceedings are expected to last at least another week.
Mr Jackson, 45, was arrested last November and charged by the District Attorney with seven counts of committing lewd or lascivious acts on a child aged under 14, and two counts of administering an intoxicating agent to the child. The singer has pleaded not guilty on all counts and called the charges a "big lie" concocted to extort money. He was released on $3m (£1.63m) bail.