California requires that will be "lodged" or filed with the court in the county that your grandfather lived in when he passed away. However, you mentioned a trust. If your son is a beneficiary of the trust, he is entitled to ask for an accounting from the trustee. You should consult with an attorney about that step.
Search Orange County Court records. If you cannot find anything under his grandfather's name, ask an attorney or title company to do a property record search. If your son was named in the trust, he was entitled to information. Waiting 20 years is too long. So long in fact that you may be barred by the statute of limitations. Check court records and talk to an attorney. At least you can get some answers and put this thing to rest.
Generally, trustee's have are held to very high standards and very high standards of care. The beneficiary does have the right, in most, but not necessarily all circumstances, to demand that the trustee disclose trust-related information. At that point, if the beneficiary believes that an impropriety has occurred, the beneficiary must, to the extent possible, expressly state the nature of the dispute or impropriety. Assuming that the beneficiary has stated a potentially valid claim, it is then the burden of the trustee to refute that claim with sufficient evidence. Assuming that the trustee can present sufficient evidence tending to refute the claimed impropriety, the beneficiary will want to present evidence establishing trustee wrongdoing. The general standard of trustee care is stated in Cal. Probate Code §16040. You need to act quickly and should consult an attorney.
The above is general legal analysis. It is not "legal advice" but analysis, and different lawyers may analyze this matter differently, especially if there are additional facts not reflected in the question. The above response does not create an attorney/client relationship. I am not your attorney until retained by a written retainer agreement signed by both of us. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here