How to request access to a deceased family member’s Apple Account - Apple Support
"While we may also be able to help remove Activation Lock from devices that use your loved one’s original Apple Account, their iPhone, iPod, and iPad will need to be restored to factory settings before they can be used with another Apple Account. 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."
Basically, if the phone is passcode-locked, and you don't know the passcode, there is no way for you to get any of the information on the phone itself – even if Apple helps you get access to your son's Apple Account. As for which specific legal documents Apple requires for that, you will need to ask Apple. This is a user-to-user forum, and the people answering you here do not speak for Apple.
If your son had his phone set to synchronize data to iCloud, and Apple gives you (or the executor/administrator of his estate) access to his Apple Account, then you might be able to get data back from iCloud. (For example, if he had been using iCloud Photos, photos would be on the phone and in iCloud, and you could get them from iCloud.)