Sadly there is not much you can do for this child. As much as you care and have extended yourself to him, he has parents that are responsible for his welfare. The fact that his mom isn't that great of mom (neglecting cavities?) is a sad reality of this little guy's life.
The thing is, his life will probably always filled with this level of drama and disruption because that is how he is being parented. It will be like plugging holes in a leaky damn if you become overly involved because the basic structure in his life isn't sound.
I know you are sad but the time you help him does make a difference, and even if you don't see him for however long or disengage, you DID have an impact.