To answer your stated question: the only general requirement for your authorship on a paper is that you contribute to the research significantly, and that generally doesn't imply co-writing it — contributing to the ideas can be significant, especially if the proof becomes trivial with an idea you contributed. (I've heard mathematicians describe "significant contribution" very differently — say, that PhD students are expected to draft papers, get advising and feedback from their supervisor, and not list their supervisor as coauthor — unless the student failed to contribute to the project).
Now, you have similar ideas. In my experience, tracking down who originated an idea among people who did work together can be very hard, and joint credit can be an easier solution which is accepted in our community (in Computer Science, Programming Languages).
However, since all authors are jointly responsible for the claims in the paper, I've been taught it's very bad style to add somebody to a draft without his permission; submitting the paper without your knowledge/consent would be unethical, but dropping contact is not only brisk, but ambiguous. Couldn't he genuinely (in good faith) think you were okay with the draft, but didn't have time to respond?
In fact, if he thinks you contributed to the research, to publish it ethically, he'll need either your agreement to have your name in, or your agreement to have your name out.