If your AutoCAD 2009 is vanilla and previously had its own license file, then you need to combine its INCREMENT/PACKAGE information into your new one. If your AutoCAD 2009 is part of the Inventor Professional 2009 (MDT 2009) then it should be able to use the AIP2009 license as part of your AIP2010 PACKAGE.
Your AIP2010 license file under Subscription will allow you to run the entire suite of Inventor Pro and Mechanical going back to 2008. However, this will not work for plain vanilla AutoCAD if it was not originally part of this suite. This may be where your boss' confusion is and why you are receiving the -18 error.
Your Network License Manager also needs to be at least version 11.5 to support 2010 Subscription licenses. This can be found on any of the 2010 Autodesk media discs.
Let me know if you have any more questions.
Good Luck!