Adult adoptions are allowed in CA, but just because you adopt him which is likely going to be allowed, it does not necessarily mean he will be able to qualify for resident alien status or citizenship status since he is an adult being adopted by US citizens. Thus, if the purpose is to try to get him resident alien or citizenship status, you may likely not accomplish that goal or it could take many years to have it approved because USCIS investigates those cases very thoroughly (sometimes their investigation can take several years).
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