Under Washington law, the amount of restitution ordered by the court
may be determined by the amount of the defendant's "gain" from the crime or the amount of the victim's "loss" from the crime. (RCW 9A.20.030)
It sounds as though the court determined that the total loss to the victim was $20,000 and, since your son was the only person convicted, required him to make the full amount of the loss, even though he didn't personally "gain" all of it himself.
I have long had a personal motto which I wrote as a young district attorney in Orange County, California, more than 30 years ago, which reads:
"No man has greater courage, honor and integrity than he who forthrightly accepts responsibility for his actions, regardless of the consequences."
I therefore commend your son for his accepting responsibility for his actions and assisting in the resolution of this crime. Doing so demonstrates that he has learned from this mistake and is not likely to repeat it.
Court orders of restitution are appeable, if the appeal is filed in a timely manner. If it is not already too late and he wants to take this route, your son should ask his public defender to file a notice of appeal. The public defender's office will not handle the appeal, but Washington is one of several states which pays for indigent appeals directly. Contact your local county bar association for information on how to obtain this assistance.
He can also ask the trial
court to reconsider the amount of the restitution fine. He can discuss this with his public defender.
The terms on which he will actually have to make payments on the restitution fine will be determined by his probation officer once he has completed his jail term.