I would suggest that both he and his mother find grief counseling. His mother needs to build a life of her own, just as he does. It is understandable that they feel screwed up, but this is rather extreme and really can only be dealt with via professional help.
You need to know what you are, and are not willing to be OK with, draw those lines in the sand, and stick to your boundaries. Even if it means ending the relationship with him.
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."