My son passed away recently, and need to unlock several Apple devices before deleting his account

For information about how to request access to a deceased family member's Apple accounts, and through this their equipment, click here --> How to request access to a deceased family member's Apple account - Apple Support

The alternative if you have the Apple receipts: Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. Do this online at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not state what currently constitutes proof. Prior to this new online service Apple typically required the original receipt issued by Apple or an Apple Authorized Reseller and documented provenance (so typically it would be the original owner who does this, or a descendant with legal proof of ownership).

Apple recommends you "Keep your receipt after you buy an Apple device" --> Keep your receipt after you buy an Apple device - Apple Support

Per the article you are referring to it says "if you need additional help, please contact Apple Support." Your options will be to call or chat Apple, a call is probably best.

After opening that link you'll want to select Other Apple ID Topics > Delete Apple ID > Continue > scroll down and select the Call option. Then you'll be connect with an Advisor who can help answer all your questions.

Again, you need his Apple ID and password before you can reset his devices. Without those, or access to his accounts, you can try to activation lock removal service I described earlier.