don't want iphone photos on ipad

Sharing an AppleID between users is not advisable. Each User should have their own AppleID - and this should be used with their own devices only.

Apple ID - Official Apple Support

You and your Son should not be using the same AppleID - as doing so will result in unwanted information sharing, privacy issues and potential AppleID account lockout. It follows that your Photo’s (and other personal information) and Keychain will potentially synchronise with devices that use your personal AppleID.

If you wish to limit synchronisation of Photos, Calendar, Contacts, Keychain etc - you can limit this on a per device basis by disabling individual iCloud services:

Settings > [Your Name / AppleID] > iCloud > [Apps Using iCloud] - deselect services that you do not wish to sync’ with iCloud

After separating your Son from your own AppleID, you might consider using the Family Sharing features to selectively share paid Apple services, Apps, Subscriptions and Media:

https://www.apple.com/family-sharing/

Set up Family Sharing – Apple Support

Share App Store, iTunes Store and Apple Books purchases with Family Sharing – Apple Support

Make sure that your son's iPad is not using the same Apple ID as you are. They should be different. If necessary, you can use Family Sharing to allow sharing of content between the two devices. Family Sharing and Apple ID for your child - Apple Support

You can go into the settings, and iCloud and turn off iCloud syncing