Hello,
I am very sorry to hear about this loss to you and your family. Unfortunately, if he had not set up the Legacy Contact feature you would likely need to seek out legal options to gain access to some, not all, of his Apple ID data. The specific device is not able to be unlocked and the passcode is not able to be accessed, even by Apple.
How to request access to a deceased family member's Apple account - Apple Support
"How to request access to a deceased family member's Apple account
This article explains how to request access to or delete someone’s Apple ID and the data stored with it after they have passed away.
Every day, people all over the world save important documents, memories, and more on their Apple devices and in iCloud. At Apple, we consider privacy to be a fundamental human right, and our users expect us to help keep their information private and secure at all times. In the unfortunate event of a customer’s death, Apple provides options for their loved ones to request access to or delete their Apple ID and the data stored with it.
While we may also be able to help remove Activation Lock from devices that use your loved one’s original Apple ID, their iPhone, iPod, and iPad will need to be restored to factory settings before they can be used with another Apple ID. Please note that devices locked with a passcode are protected by passcode encryption, and Apple can't help remove the passcode lock without erasing the device.
For security, Apple requires and verifies legal documentation before we can assist with a deceased person’s account. This generally includes a death certificate, and may also require a court order or other documentation. The requirements vary by country and region. We have great sympathy for surviving family members and try to help with requests as much and as quickly as possible."
The above article goes on to specifics about the Legacy Contact feature and other options for requesting the access. The below article shows what kind of data can be accessed.
Data that a Legacy Contact can access - Apple Support
I hope this is helpful and, again, am sorry for your loss.