Do you mean his user account with Administrator Rights? you can remove the password. Open windows Control Panel and in the top right corner of this window, next to View by, select Large icons from the dropdown box and then open User Accounts. Click the link 'Remove your password', enter the current password and then click the 'Remove Password' button.
Next you could create your own user account with your name and Administrator Rights and then disable his.
If you mean the built-in Administrator account (usually hidden to protect it) then post back for advice.