No money could be forwarded until there was a court order allowing it to happen. The attorney has no authority to do so. Someone should have filed a petition to probate your mom's estate. If the attorney did not properly do this for you or a sibling, then the attorney has committed malpractice and could also be disbarred.
This is practically a tough case. Proof is going to be difficult and it sounds like the attorney did not keep good records and even if he does have records, if he is unethical, it may be difficult to get your hands on them. I assume the attorney was her PI attorney and had the money in trust which is how he had control. He certainly should not have distributed after your mother's death without the propery authority. Many, many issues here and you should speak directly with an attorney to see if it is worth pursuing.
You can have a oral trust in California over personal property. That is what that attorney is going to claim. The oral trust would be for the specific son. As Ms. James points out this is a proof issue. The specific son could sue for an accounting of the oral trust and seek damages for mismanagement. The attorney has very heightened standards if he was hold the clients money. Cash disbursements are a convenient excuse. A state bar claim may also be in order.
One way to force all the issues would be to open a probate and sue the attorney under Probate Code Section 850.
The general advice above does not constitute an attorney-client relationship: you haven't hired me or my firm or given me confidential information by posting on this public forum, and my answer on this public forum does not constitute attorney-client advice. IRS Circular 230 Disclosure: In order to comply with requirements imposed by the Internal Revenue Service, we inform you that any U.S. tax advice contained in this communication (including any attachments) is not intended to be used, and cannot be used, for the purpose of (i) avoiding penalties under the Internal Revenue Code or (ii) promoting, marketing, or recommending to another party any transaction or matter addressed herein. While I am licensed to practice in New York and California, I do not actively practice in New York. Regardless, nothing said should be deemed an opinion of law of any state. All readers need to do their own research or pay an attorney for a legal opinion if one is necessary or desired.