Imposing your standards on your parents may backfire, so take some time to step back from the situation and think about your approach. It's important to remember that people have the right to refuse help and make bad decisions, as long as they have the capacity to understand the consequences. As a son or daughter, you can talk to your parent about your concerns, but you can't force him to change. If you believe that your parent is truly at risk, then start small. Requests or demands for sweeping changes will inevitably be met by resistance.
Assertive communication is another good strategy.
I worry about the choices you are making, and I would like your permission to talk with your doctor. Maybe the three of us could sit down together. If your attempts to help continue to fail, then accept your parent's decisions. Many experts think it is more important to maintain a relationship with your parent than it is to win an argument. However, respecting a parent's right to choose becomes more difficult as he enters the gray area between being fully capable, intermittently confused, and the early stages of dementia.