What can i do if my step dad is addicted to diet coke is there any long term diseases i could scare him with?

Compounding insults: We are programmed to live to 120. The reason we don't is because of the toxic abuse we put our bodies through. Consider the long term effect of the chemicals in your dad's addiction compounded over the years. His roi (return on ingestion) may be a penalty for early withdrawal from his life account. I see that you care very much for him. I hope he knows how much his habit will hurt you both.