How to ask a professor if s/he is interested in being a coauthor on a paper?
Should I just ask him at the next meeting, "Would you be interested in being a coauthor?" Is that the typical way this conversation goes?


In short, yes. "I think you contributed valuable ideas to the paper. Would you like to be a coauthor?" would also do. If he rejects, you can insist, say, two more times for politeness reasons, and if he still rejects, go on without him. But also be prepared for an answer "it is too early to speak about my contribution; I did not do too much yet". The deal is: you would be trying to make him work on a paper (via writing, discussions, or by providing ideas) and would offer him a place among the coauthors commensurate with the contribution.