Please calm down. You don't know how he will handle this. Please, please don't reach out to girlfriend....she told him she wants a wedding and kids so he knows the problem and she doesn't need or deserve pressure to stay with him out of guilt. It is his issue, not hers. She is a actually making a healthy decision for herself....please don't guilt or beg her.
Having said that, I can only imagine your fear. But he may do better than you fear. He clearly doesn't want what she does. If you are there you can maybe try to gently encourage him to get psychiatric help and rehab. in my opinion these are what he really needs to start healing. Nobody can heal him but himself. No you can't.
I hope you decide to get therapy for yourself to learn coping mechanisms and how to handle issues with your son. Wishing you peace, love and light.