If you receive a subpoena, you should obey it, show up, and testify with the truth, the whole truth, and nothing but the truth. If you don't receive a subpoena, you don't have to do anything. Whether your boyfriend will be released if you do not receive a subpoena and testify is not possible for anyone to know in advance.
You might want to consult a criminal defense attorney about this. One who knows all the facts could have a very different and much more insigtful answer to your questions.
Good luck to you.