I want to remove my payment on my sons iphone

If this is done as part of Family Sharing and you are the Organizer, you cannot.

Ref:

- Family purchases and payments - https://support.apple.com/HT201079

- How App Store and iTunes Store purchases are billed - https://support.apple.com/HT201359

- Check your Apple ID balance - https://support.apple.com/HT202359

If you set it on there, this document explains how to 'Change, add, [review,] or remove Apple ID payment methods' on an Apple mobile device, a Mac, or a PC; click here --> https://support.apple.com/HT201266