Well she can't press charges; she can only report the incident which she must have done. It is the decision of the DA to proceed with prosecution or not. If your son posts bail he'll get out for sure, probably subject to a protective order forbidding him from contact with her. His lawyer may be able to get the bail lowered or even to have him OR released but that would be very unusual in a DV case. These cases are usually prosecuted very aggressively.
It is the DA's decision to prosecute. The fact that the victim does not want to proceed will be dealt with by the DA. Your son needs an attorney and will probably have to post bail.