It would depend on where he's incarcerated. See if you can find a web site for the governmental jurisdiction's jail or prison system and there's probably a contact number there. Or, if he's local to where to live, look in the government listings in the front of your phone book.
It is hard to get the number for the facility that he is in but you can get in touch with your boyfriend's lawyer. He is in a better position of getting in touch with the prison authorities to find out when your boyfriend's release date is.