US Citizen filing Greencard for father (mother expired)

Hi experts,

I am a US Citizen residing in California. My father is here with me on Tourist Visa (validity till 2028). My mother has expired and I want my father to stay permanently with me. My father and mother never registered their marriage in 1970 and therefore he doesn’t have a marriage certificate. Could you please tell me what documents I need for filing his greencard, and if marriage certificate is mandatory. I would like to file his greencard before his current stay expires. Thanks a lot for your help and advise.
 
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