I really don't know the right thing to do in this situation....but if you are just going to kind of "fade away"...maybe send a nice letter telling him all of the good things about working with him, the way he has helped you, the things you've learned, how you've grown?? The situation now, even though it's really sad and hard, doesn't negate all of the hard work leading up to this point.

I'm sorry you have to deal with this....it sounds really hard.