If you do not know the passcode, there is no way into the phone without erasing everything on it – including any photos stored on it.
Request access to a deceased family member’s Apple Account - Apple Support
"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."
Hopefully the photos exist elsewhere, too. For instance, if your son was using iCloud Photos, any photos that had been synchronized to iCloud would survive the resetting and erasing of the phone. Assuming that Apple granted access to your son's Apple Account, the photos could be obtained from there.