Two options:
1. Create his own profile and then trade to him all his cards.
2. If you want him to use your ID, under Profile, go to "Collection". There you will see a large green button "Create a New Collection". Click on it and name it "xxxxxx". Then you can move all of his cards (set by set), into this collection that will be for his use.
Bruno
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Check my Profile page to see my 2023 Goals and my Lists of sets near completion (5 cards or less) or sets getting close (less than 100 cards missing and 75% complete).
https://www.tcdb.com/Forum.cfm/Page/B/ID/0/?MODE=VIEW&ThreadID=25745&C=0