Anyone trying to get him to think in rational terms is going to end up frustrated. He has a legitimate addiction to food and soda that is a diagnosable disease, along with mental health issues that compound and intensify his addiction. His most telling response was when he said he wouldn't quit soda and basically said it was the only enjoyable part of his life.
His reward centers for dieting were a bunch of people cheering him on, but as with anything on the internet, there is a pretty severe diminishing return where the support turns from highly motivating, to just the same sad echos he hears every day. At the end of the day, in order to do something painful, you need long term justification or vision for why you are doing it. Frank's motivation was all short term- dopamine bursts from people cheering him on. Based on his tweets I don't think he conceptualizes the incredible things that await him if he can erase his addictions. The long term benefit does not even come close to out weighing the short term immense pain.
He needs to realize life isn't and will never be fair, and he has to demand something better for himself. I really wish all the best to him, it's hard to watch people struggle.