You are between a rock in a hard place for sure. I heard it from my nutrition coach a long time ago and I have almost (almost) always found it to be true. He said "you can change almost anyone, even total random strangers literally just pulled off the streets but the hardest, most stubborn people to change, is your own damn family."
I have very rarely seen this not to be true. The other family member may get dragged some where, to a nutrition lecture, diet-guru, doctor, super markets - make promises and swear only to return to normal habits within a few days or worse hours.
I know this sounds like a "no hope" reply but that is kind of the way it is. When I watch shows like Intervention I see a lot of similarities of people with food and diet. They wont change unless they want to change. You cant make them. He has to do it for himself. You can sit there and cut your heart out and bleed it dry for him and it wont do anything. Ive seen people lose family members this exact way.
If you can try to find out what provokes him. Why does he eat trash? Did something happen to him a while back? Why wont he change? What can possibly make him change? Does he even know what this is doing to him? Do you even know? Who is going to take care of him when he is sick? Then really sick? Then hospitalized.
Watch the documentary Sick, Fat and Nearly Dead with him. See if it strikes a cord. Is he depressed? On medication (of any kind?)? Is something else making him fat?
Some people you have to hit them just the right way, with the right string of words, thoughts and ideas in order to produce change.