What happens if he doesn't respond?

I am amazed you were able to obtain a writ of mandamus, as judges rarely grant them given that no other remedy at law must be available in order to justify the use of mandamus, from the Latin "we command." However, since you did prevail in your petition for the writ, and a deadline was set for the judge's response, general rules of civil procedure require a respondent to give a formal reply within the stated time frame. Therefore, if the respondent judge failed to give a reply you could either be granted an order of court -- a default victory for lack of opposition -- or the respondent judge could be held in contempt of court.