I change Apple ID of my son, how can I delete and remove tho old Apple account !!

Hi Dndnmms,

Welcome to the Apple Community.

We are all Apple users, like yourself, in this community, so cannot speak for Apple.

Seems you can delete your son's old account,

yet be aware any purchases from App Store, Music, Books etc, made on that account may be lost

Complete details are in the Apple Support article(s) below.

How to delete your Apple ID account - Apple Support

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Another option is to deactivate the old account.

(If needed it could be re-activated if you keep the access code that you'd need to use.)

How to deactivate your Apple ID account - Apple Support

All the best :-)