There's an ICE hotline you can call, for people detained by ICE who believe they are U.S. citizens. http://www.ice.gov/detention-reform/toll-free-hotline/. Be prepared with documents that prove his citizenship, like his birth certificate or passport.
The information offered is general in nature and not meant to be relied upon as legal advice. No client-attorney relationship is created through this information. Please consult an attorney prior to making legal decisions.
ICE does make mistakes quite a bit about who they detain. If he was born in the United States then get a certified copy of his birth certificate to ICE as soon as possible. Think about speaking with an immigration attorney to make sure everything is done correctly and quickly. Good Luck.