Signature question

I am trying to set up a workbook in Excel 2007 with multiple tabs and two signature lines at the bottom of each sheet. The idea is that my boss can read, sign and close my work as I complete it.

The problem is that each time you sign a tab/sheet, it converts the whole workbook to read-only.
I only want one tab/sheet as read only. In other words, as I complete Sheet A, he signs it and it converts Sheet A to read only, leaving B through Z open for additional work.
It will just be too much to have 100 separate workbooks for us to keep track of for this function.

I need any ideas I can explore. Thanks.