My Boyfriend is Bulemic.

He recently opened up to me and told me that he has been throwing up after meals. I asked him why, and he says it helps him lose weight (although he's very thin) and it helps him relieve stress. He said he's been doing it for a few months, and he can't stop. When I asked him if he wanted to stop, he replied with, "I don't even know. It works for me." HE won't admit that he's bulimic. I expressed my concern for him and my disapproval with this, but I know I can't force him to stop. He is afraid that I'm going to leave him because of this, or I'm going to tell someone. I'm trying to be supportive and loving, but I'm scared. No matter how much I convince him that he's perfect the way he is, he still thinks he needs to do this.
My problem here isn't how to help someone with bulimia, its how to help someone realize that they need help. I want him to get medical and professional help. How should I go about this without breaking his trust?