Robert Downey Jr., 35, is due in court in Indio, Calif., today to face arraignment on drug charges stemming from his Nov. 25 arrest in Palm Springs. At the younger Downey's request, the actor's father, filmmaker Robert Downey Sr., 64, has flown from New York to be at his son's side, a family friend has told PEOPLE.com. Downey Jr.'s lawyer, Robert Waters, is mounting "a vigorous defense," Waters told USA Today. "We're going to challenge everything that is possible," he said. That possibly includes challenging "one of the counts during the arraignment," Waters said without elaborating. Describing his client's mood (Downey reportedly checked himself into L.A.'s Cedars-Sinai Medical Center three weeks ago for an intense 72-hour depression treatment), Waters said, "He's had his ups and downs . . . doing the best that he can under the circumstances." Downey is charged with felony possession of cocaine, felony possession of the prescription sedative diazepam (also known as Valium) and a misdemeanor count of being under the influence of a controlled substance. If convicted, Downey faces a maximum of six years in a state penitentiary on the felony counts, a year in jail on the misdemeanor, and a possible extra year behind bars because of a previous felony conviction.