Question: My son has been waitlisted by the University of Michigan, so I would like to know if Michigan is accepting applicants off the waitlist this year. Also, when do they start taking people off the waitlist?
This is a question for U. of Michigan, not "The Dean." You should feel free to contact the admission office and ask about waitlist plans. You may get an answer that isn't completely specific (e.g., "We do expect to make some offers of admission") rather than a very clear one ("We have 17 places open right now") but at least the response you receive should put you in the ballpark.
In general, there can be wide swings in waitlist activity from year to year, even at the same institution. In other words, in some springs, a college may accept more than 100 waitlisted candidates, and then the next spring , it's zero. But this is definitely a question for the college itself.
Sometimes colleges start using the waitlist even before the May 1 "Candidates Reply Date" when their enrollment return cards are trickling in too slowly for comfort. Typically, however, the serious waitlist action starts right around now (a few days after May 1) and continues through the end of June. Some colleges may keep a few students on their waitlist through the summer, but usually by early July, most applicants will be told to stop waiting.