Since the DA didn't file charges within a few days as required if your boyfriend was in custody, they had to let him out of custody. That doesn't preclude the DA's office from filing charges in the future. They have a year from the date of the incident if it's a Misdemeanor filing, and generally 3 years from the date of incident to file if it's a Felony filing.
Technically you are correct that if no charges are filed within 72 hours they are suppose to release the accused. However, they can "re-file" and keep him in custody. Unfortunately, there is no clear remedy for this situation. However, since it does appear that the DA intends to file charges, getting an attorney to assist with bail and defense against the charges is well advised.
Response to answers is for general information purposes only and does NOT establish an attorney client relationship. Further, without a full client interview, opinions are based on limited information and therefore a approximation. Actual advise may vary significantly once addition information is established.