My partner of 8 years tested positive a month ago. I am negative. What can I do to help him deal with the guilt and dread that he is experiencing? What sorts of things do I need to make clear to him so that he begins to feel better?

Very little of this is in your control. It takes different people different ranges of time to cope with these feelings and concerns. Time generally helps. Being present and supportive is helpful. Remaining open to talking about all of the issues is important, however, I think you need to also be careful about not "pushing" him. Again, everyone has a different pace with how they process their emotions over something like this. If he and/or you have very prolonged difficulties with any of this, I suggest you consider some counseling from a mental health care provider with experience in HIV related issues.