Delete folder in Macintosh HD

I have bought my nephew Mac Pro. I have deleted his account and changed to my apple ID. Under Users & Groups no longer has him as administrator. However, whenever I download files it will go to a folder that is under his name, I cannot find his folder on "Finder" unless i search it then it will appear under "Others" and on details it shows that this folder is in Macintosh HD. Another annoying matter is, it will appear a window from Apple ID to ask for his password. I think I have deleted and/or changed everything that is under his name. When I proceed my community question, on step 3 still shows "Diamantino's MacBook Pro". This is pretty annoying. How can I make this MacBook Pro "officially" mine?