I am a U.S. citizen and he hasn't left the country. The main problem for my husband and I is that the prostitution is listed as a crime of moral turpitude and there is a question on the paperwork saying if you have been convicted of a crime in the last ten years so we don't know whether or not we need a waiver since it has been ten years.
Generally speaking you have to disclose any arrest when asked by CIS, as expungements usually are not recognized for immigration purposes. One conviction may not be fatal to your husband's case but TWO crimes involving moral turpitude would most probably be. I would suggest meeting with counsel.