What can I communicate with a person who is sensitive to negativity in a disagreement?

I know someone who is extremely sensitive to stress and very sensitive to a negative tone of voice. Sometimes during a disagreement he tells me that my communication is having a harmful impact on him. Once he believes I am being negative I can't seem to find a better way of communicating. In fact, it is often a self fulling prophesy because his accusatory manner becomes unbearably frustrating. What advice might you offer?