How to transfer my sons xbox account to his own microsoft account

My sons xbox account is under my microsoft account, as it was setup when he was young. He now wants to transfer this to his own account. I cannot believe the lack of foresight that nobody at Microsoft has thought this might be an issue. There has to be a way of separating the accounts? Is the only option for me to give him my microsoft account by editing the contact info? Then I open a new account but what happens to one drive and my office 365 subscription? That just renewed 6 weeks ago for another year. He's 16 now what happens when he leaves home as an adult, will he still be tied to my microsoft account. This is beyond ridiculous that there is not a simple solution for this issue as I know its a common problem.

:)