I went through this years ago as the son. I needed the next month to clean out the apartment. I found the money. I think I might have charged it to my credit card. PayPal would work today.
When is the lease up? Is there a clause that handles breaking it early?
I'd probably let the son stay until the end of August, in hopes of not having to deal with the belongings myself, but I would charge ample late fees for paying late, and maybe insist on weekly progress payments.
Since you are in California I have no idea but you should consult with a good attorney who specializes in landlord-tenant law specifically. Not just real estate law.
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