Estate Law Questions? Ask an Estate Lawyer.
As an aside, you really don't need to open a new question each time, as the JustAnswer folks charge for each question, and you can just reply in your other questions and I will respond..
If you aren't on good terms with the family, then once a probate case is opened and you are appointed Administrator, you can subpoena documents from the wife or anyone else who may have them.
But a 401K goes to the wife automatically under federal law as the spouse is the default beneficiary unless they specifically waive those rights in writing. So that wouldn't be an asset that would be in his estate as it passes directly to the surviving spouse just like life insurance goes to a named beneficiary.
But as a child no, you don't have a legal right to his financial records. As the Administrator of his estate, you would.