If he died in the US, you can call the city / town clerk of the place he passed and ask how to order a certified copy of the death certificate. Some places require that you provide a reason for your request. You have a good reason. There is likely a small cost for the certificate, but it shouldn't be too difficult to obtain. If he died in another country, you should be able to find out how to get the certificate (if you speak the language) in a similar way - contact the local record keeper. Good luck -
Please note: The above is for general information purposes only. It is not intended to establish and does not establish any attorney-client relationship.
I agree with the attorney above. Call the vital records department or appropriate government office of the county or city where he passed away. They should be able to provide you with a form to complete your request. A quick google search may even reveal that you can fill out a form online.
Good luck with your search.
This response is for informational purposes only and is not meant as legal advice. No attorney-client relationship has been established.