What do I do when a co-author takes too long to give feedback during the peer review process? Two years ago I did a piece of research for a journal's especial edition. I got the reviewer's comments, did all the corrections and sent it to my co-authors. One of them took so long to return his comments that the paper wasn't included in the EE. I then tried to submit it to another journal but again the same co-author took long time to provide his comments. Finally my boss suggested me to submit it to another journal (good one) without waiting for my co-author's opinion. I got the reviewer's comments back, I did all the corrections (I have 45 days), sent it to my co-authors and gave them a week to send me their comments. The same co-author is now telling me he's not happy I didn't tell him I submitted to that journal and that he won't be able to make comments in a week. I'm again in a catch-22. What do you do?
It seems you are following your boss' (good) advice. Dealing with co-authors is not always easy as you have experienced. I think I have experienced a similar co-author, not responding but complaining whenever something happens. In my case, the co-author was definitely over-committed and the actions were basically a symptom of frustration about not being able to perform. Now, I think you have done the right thing to a point. The only thing that would have improved things would have been to provide the co-author with the information but with a strict deadline but not more than a week or two. Doing so at all stages, is the only way to deal with such cases. It is not very nice, or comfortable, but it is necessary to make a stand and convey the seriousness. In your case you have clearly had a lot of extra work due to the (lack of) actions from your co-author. I assume he has not apologized?

So what to do? Well, I would write and state that you are sorry the co-author feels left out (or however the co-author has phrased it). You can then state that the lack of response earlier made you assume the co-author was not prioritizing the work and that based on suggestions from your boss you have now taken the actions you have to get the material published. Be brief and courteous but do not add many excuses.