What's to Do When there is Nothing You Can do To Save Him

Mu boyfriend and I just spent the last 3 hours fighting. Not over some stupid shit but something serious. He's going to run away from home. He's 17 and he hates his home life doesn't see him getting very far in life. He has a small chance of graduating high school. Getting his GED is even worse. Now he thinks he can survive on his own amongst the streets of West Seattle. The bad side of the city. I told him I'm breaking up with that there was no point on keeping this relationship up if he was actually going to do it. He agreed and now we are officially not together. So here we are. Me crying in my room writing this journal that probably means nothing to you people. I've tried everything to keep him from doing it bu he's stuck on it. I still love him and I don't want to give up so easily. I've considered texting his mom to tell him but then he'll go to counciling claim him crazy for his terrible terrible bipolar and he'll go to a mental hospital where he'll hate me for the rest of my life. We've been together 3 years now it shouldn't end this way and his life shouldn't end with him as a low life on the streets. This has been something he's been juggeling around since 6th grade he's set and I can't let him do this. What do I do when there is nothing I can do?