You should be able to simply disconnect from the project.

When you reconnect, do so with his login credentials.

1) Open the BOINC Manager
2) If it is in "Simple View" click on View->Advanced View
3) Select the "Project" tab
4) Select the "World Community Grid" Project
5) Click on the "Remove" button (Left side under "Commands"
6) Confirm the action

You should then be detached from the WCG Project

Then select Tools->Add project or account manager

Select add project and scroll down to World Community Grid
Select it
Sign up for WCG with you user name

Would be a good idea to first set 'no new work' and run the host dry before doing the remove/re-add cycle with the son account.