You can tell him you have our permission to think of it as 5.0 instead of 05 if that helps.
All our releases will look like guava-r## ... Unless we actually release monthly for 8 1/3 years, in which case we'll get to guava-r100 and really screw up the whole lexicographical order thing.
What exactly does your PM mean by "not stable"? It's not a great idea to assume anything positive or negative from a version number. Tell us what kind of assurances he's looking for and we'll try to answer for those. "Stable" doesn't mean much (after all, the deprecated library is as stable as it gets, because we're never going to change it!).
(Note that questions like this, which are not "how-do-I" or "why-didn't-it" questions, are welcome on our mailing list (linked from our project page at http://guava-libraries.googlecode.com), while I'm actually not sure whether the stackoverflow community considers them entirely appropriate.)