You can't take away his pain. That's how we learn and grow....and become better partners. You can't call the girl and you can't put her down. That will put a distance between you and your son. He will want to defend her and that is the LAST thing you want him to do.
Ask him if he wants to really be with this girl or just be in a relationship. Ask him to write on a list the qualities of what he wants from a girlfriend. What are the things he likes about this girl? What are the things he wished he could change about her? Tell him he DOESN'T have to share this information with you. It is just for him to reflect. Ask him how you can help (change phone numbers, help him respond in a positive way to her texts, find a counselor, etc.) Ask him if he has a close friend that he can share his feelings with. If you know he has a good friend, ask him if he is sharing his feelings with this friend. Ask him what his friends think of this girl (don't add your commentary or thoughts). The more you can make HIM reflective on the situation, the sooner he will see how bad it is for him. He has to make the steps towards a healthier relationship with this girl.....even if it means he continues dating her (I know...not what you want to hear). And lastly, I've learned that if a teenage boy wants to talk...LISTEN without judgement and try to ask the questions a counselor would ask (without getting worked up).
Remember, this is a developmental milestone for teens. They are learning to become independent. As much resistance as you get, remember it is a natural phase that is healthy. They have to make their own choices and learn from them. This part really sucks for the parent! Good luck with this. It is obvious you are in pain for your child. That is love.