This is a very good question. ICE (Immigration and Customs Enforcement) has been recently under fire for their poor "transparency". Unlike other law enforcement agencies that are compelled to release such information, ICE often operates in secret. Usually, relatives try to contact ICE to find out whereabouts of their loved ones. The website with info is www.ice.gov. They have a toll free number to call.
However, you are not technically a relative and this can cause problem. Your best bet is to hire an attorney for your boyfriend and such attorney would locate him for you and would try to help him.