How can I get in my deceased son’s phone without losing his pictures etc

I'm sorry for your loss.

This is Apple's procedure for requesting access to a deceased family members device --> How to request access to a deceased family member's Apple account - Apple Support

Sorry for your loss.

Unfortunately without the phone passcode, there is no way to get into the phone without restoring it and erasing everything on it.

If he was backing up regularly or using iCloud, acquiring access to his Apple ID, would provide access to backups and synced photos at https://icloud.com