Is there any reason an executor can keep a sole beneficiary of a house from living there after the parent dies?

No, I don't see any reason that the nephew should be required to leave, particularly if the contents were gifted to him as well as the home. In these times, any property is apt to suffer loss if unoccupied. In California if he had been living there (particularly if he received his mail there) he counts as a tenant under the law, and is protected by landlord-tenant law, including the Covid-19 extra protections. He does need to allow his aunt into the home to look for documents, if she requests and gives reasonable notice. You use the term "executor". That implies there is a will, which will have to go through probate. During the time probate is happening, the executor is legally responsible for the home. The aunt may just be nervous about having responsibility for something when someone else is living there. The estate is responsible for maintaining the insurance on the home; your nephew is within his rights to require proof of insurance. If he's living there, it seems reasonable to me that he should repay the estate that cost. I would suggest that they jointly seek an hour or so of counsel to reassure both of them. It might be beneficial to have a short agreement written up covering their respective duties and responsibilities. (Most attorneys are doing Zoom meetings in these times, so it's no barrier that the sister lives in Washington State). If the property is in probate, and the aunt has received her letters (authority from the court to act), she can officially distribute the contents of the home to the nephew. Probate Code Section 10520 gives authority for distribution of "household furniture and furnishings, motor vehicles ... and other items of a personal nature to the persons entitled to the property under the decedent's will." Your nephew can then sort, discard, clean, etc., to his heart's content. He would insure the contents under a renter's policy until the house is his. If, in fact, the property is actually in a trust, the aunt can and should immediate deed the property to her nephew.

While the father may have specifically gifted his house and contents to his son, if the other property he left is not sufficient to pay his debts, the house must be sold to do this.  In those circumstances, it is the executor's duty to remove the son from the house and sell it to be the debts.