It's as if he can't make the connection between diet, his health condition, and the inevitable consequences, which are serious. He might agree but there is huge cognitive dissonance, because disease is a slow process and people never seem worse than they were the day before. That and some people hate having to be prudent and denying themselves something simultaneous. But it's, like, serious. So I hope you get through to him.
Just a quick literature search pulls up some hard evidence. Sugar-sweetened beverages impair glucose and lipid metabolism, cause inflammation and shift the lipid profile over to a more atherogenic orientation Low to moderate sugar-sweetened beverage consumption impairs glucose and lipid metabolism and promotes inflammation in healthy young men: a randomized controlled trial Randomized, controlled trial, good quality and everything. What more does someone need to know? If you read him the abstract and he doesn't take it seriously then I have nothing.
I think that you should get him on a good quality magnesium supplement ASAP. How hard would it be to get him to take some magnesium every day? It would be almost guaranteed to lower his fasting glucose, magnesium deficiency is one of the major factors in insulin resistance. Magnesium Intake in Relation to Systemic Inflammation, Insulin Resistance, and the Incidence of Diabetes That will help. Vitamin d as well.
Stabbing conventional wisdom in its face.
Anyone who wants to talk nutrition should PM me!