Has he been checked for Hep C just a thought
The only way to change someone so stubborn or fearful of findings, is to deliver an ultimatum...go or else I'm done because I won't watch you kill yourself (possibly) with something that could easily be diagnosed with testing and that won't be done without a physician ordering them. Seems harsh for sure, but it could be so many things wrong that he needs to have some diagnostics done (lab work, CT or ultrasound, etc). Good luck to you and him. Perhaps going along with him for support is going to be helpful.