Depending on what your son used as settings/features and how much you know this might be very difficult to achieve and even have a chicken and egg problem preventing you from getting in.

I'd start with simple things

• I suppose you do not know the PIN code of his phone (don't try to guess, you might trigger a wipe of the phone)
• Do you know what email address he used of his AppleID ?
• Do you have access to that email ?
  • If you have: that's a good starting point to expand what you can access.
• Did he make backups of his phone on iCloud ? On a Computer ?
• Did his phone automatically upload his pictures to iCloud ?
• If he has a mac, his photos might be synced to the photos app

So, I'd start making an inventory of what you have/know and how you can expand that access to get to what you seek to recuperate.

Be prepared that breaking in into an iPhone is a pretty hard task if you know nothing, and that iPhones can be set to wipe themselves (permanently) if you try too much PIN codes to get in.

Apple might help, but purely breaking into the phone: they have precedents where they refuse to do so even for law enforcement, even when the user was a terrorist. Not sure how they'd react to a parent with a certificate, nor how impossible they made it for themselves to do that.