Apple

Try the “forgot your password” link on his iCloud account. When you go to iCloud.com, there’s a link there for it. There might be some way that you can reset the iCloud account and get in that way. Most of these methods have you verify you own the account like a link in an email or text to the phone or something. Worth a try to see especially since you physically have the phone.

If he left his iPhone with the messages showing on the screen even when locked, you may be lucky and get to send a text to the phone and get a code to reset the iCloud account that way.

Either that or going through Apple may be your best bet since the pics are likely backed up to his Apple’s iCloud. Apple gives everyone free 5GB so surely there’s something there even if he didn’t pay for it or it lapsed after his passing.

I got this idea from the San Bernardino shooters years ago. Apple refused to help the FBI hack into the phone but said they would help if the FBI let the phone back up to iCloud at night like this because it’d be on their servers and technically theirs to comply with.

So it seems this might be some type of legitimate way of getting stuff out of an iPhone like this by Apple’s own admission. I just checked and Apple does have an article on this exact situation and what they can and cannot do https://support.apple.com/en-us/102431

My condolences to you and your family. Good luck getting those pictures.