Adding Authors After Manuscript Acceptance I was recently informed that a manuscript that I submitted to a journal a long time ago has been accepted.

I submitted it as the sole author, however, my advisor had some comments and suggestions that I applied to the manuscript. 

As a courtesy I would now like to add him as an author. 

My question: can a second author, i.e. my advisor, be added once I receive the galley proof, or whatever that is called, to the manuscript.

Any thoughts would be highly appreciated.  

Possibly related to this question.
The difficulty here is that there's not a lot of ethical flexibility.  It's possible that this is a borderline case, where it's genuinely unclear whether your advisor should be an author and the two of you could reasonably change your minds.  However, most cases are not near the borderline.  As a general rule, either your advisor should have been an author all along (in which case submitting the paper without him was unethical), or he should not be an author now (in which case adding him would be unethical).  It's possible to change the author list after acceptance, but the editors will be suspicious and will ask for an explanation of why nothing unethical is going on.

I assume you discussed authorship with your advisor when you submitted the paper, and both of you approved of the author list at the time of submission.  (If you didn't discuss it, then you need to do so immediately.)  In that case, what has changed?  It's usually best not to revisit the authorship once it has been agreed on, but if you feel strongly about it you could explain that you feel the current author list undervalues your advisor's contributions and that you think he should become an author.  (Note that as Nate Eldredge points out in the comments, this is only allowable if he made substantial intellectual contributions to the research, not just to thank him for his supervision.)