"I'm certain one of my students has ADD, but his parents refuse to hear it. What should I do?"
Are you the first teacher to mention ADD to his parents? If so, you should know that it's going to be difficult to be the first messenger.
Things might go a bit more smoothly if you try to establish a relationship with them. Start by telling them about their child's talents and positive traits.
Then, ask questions: "What happens at home during homework time? Has your son forgotten to turn in homework in past grades?" Once the parents see you as a partner, they may be more receptive to your ideas. And remember - although you play a vital role in describing the student's behavior, only a doctor can diagnose ADD.