Unfortunately, you are going to need the email address of the Apple ID used. This is something that Apple will not be able to find for you. If you know anyone the he may have communicated with by email, it may be that same email address. In the article previously provided, along with the Apple ID email, Apple will also need a court order to allow them to provide the information to you. That information can also be found here:
In the U.S. and other locales, you can request access to a deceased person’s Apple ID and data with a court order that names you as the rightful inheritor of your loved one’s personal information. In some jurisdictions, such as France, Germany, Japan, Australia, and New Zealand, alternative documentation and process instead of a court order are accepted. Where applicable, the court order needs to specify:
- The name and Apple ID of the deceased person.
- The name of the next of kin who is requesting access to the decedent’s account.
- That the decedent was the user of all accounts associated with the Apple ID.
- That the requestor is the decedent’s legal personal representative, agent, or heir, whose authorization constitutes "lawful consent."
- That Apple is ordered by the court to assist in the provision of access to the decedent’s information from the deceased person's accounts. The court order should be addressed to the relevant Apple entity.
If you have a court order with this information, or if you need additional help, please contact Apple Support.