Hi, I'd like to help you with your question.
I understand that this must have been shocking news. As a mother, especially one who cares and is involved like you, you would feel a lot of emotions related to hearing this about your son.
Did your son's girlfriend tell you that this was told to her in confidence? If so, that makes the situation a bit more difficult to navigate, but not impossible. What you can do is ask your son's girlfriend to talk with your son about the situation and ask him if he'd be willing to share it with you. She can let him know she feels it is important you know, especially since you are very involved with his life right now.
Your son's girlfriend can also chose to tell him straight out that she mentioned the incident to you during a time she was upset about the breakup. He may be very upset by this, but he will eventually cope with it. And being honest, even when it hurts, is the best option.
Or, if this was not told in confidence to his girlfriend, then you can approach your son and talk with him. Let him know that you learned of this abuse and you want to talk with him about it. Try to use "I" statements and be non judgmental. Don't come out and say "why didn't you tell me?". What you don't want to do is put him on the defensive. Try to listen and be understanding. See how much he shares. It may not be much. He might not have dealt with the emotions around the abuse which can range from shame to fear to depression and anger. Even though he was in counseling when he was younger, he may not have shared this either with the counselor. So you will have to go slowly and see what he is willing to talk about.
Depending on how open he is to talking about the abuse, you can keep talking with him until you both feel you have addressed the problem. However, if you feel the issue is not being resolved or you have come to the point where you hit an impasse, think about seeking out counseling. If he is willing, have your son see someone with you. If not, go alone. You need the support and the guidance of someone who can help you handle this situation so you feel it is as resolved as it can be. Here is a link to find a therapist in your area:
If you attend church, you may also want to talk with your pastor about the situation. They are often well versed in helping people deal with situations like yours.
I hope this has helped you,