I understand your concerns. As to the fact that he was not given an opportunity to give a statement, that may in fact not be as bad as it seems. Many factors will be involved as to the ultimate outcome of your son's case, but depending on circumstances, your son's case has the potential for a resolution in his favor. It is important that your son is competently represented as early as possible in this process. The criminal justice system has many pitfalls that must be avoided.
Sorry your son has to face these charges, but it is not unusual for the police to take side of the person who remains on scene and is present when the police arrive. Also when police arrive on the scene of alleged domestic violence they will usually arrest the person who does not have visible injuries.
I do however, agree with my colleague, your son need not give any statements to anyone other than his attorney. At this point since he has already been charged it is most important to begin to prepare his defense. So long as your son's injuries are recorded in someway or on some documents he should have a viable defense. If you have any questions or need more information you can contact me at Debra@DebraRaineyLaw.com.
Domestic violence laws in Pennsylvania are gender neutral. Women are often perpetrators of domestic violence. You state that your son was brutally attacked by his girlfriend. He should document his injuries by taking photographs and obtaining medical treatment. You say that the detective acknowledged your son's injuries as worse than his girlfriend's. Is this stated in any police report? Your son should immediately file a PFA against his girlfriend. A PFA is a civil restraining order that would prevent the girlfriend from having any contact with your son for as long as three years. Violating any provision of a PFA could get her arrested for up to six months. Your son can file the PFA for free and would be entitled to a free attorney. As for the criminal case, your son needs a criminal defense attorney with experience handling domestic violence cases. At your son's upcoming preliminary hearing, your attorney will have an opportunity to cross-examine the police officers -- including the detective you mentioned. Lastly, if your son has evidence of being brutally attacked, he should file his own complaint with police and seek to press criminal charges against the girlfriend.