The prognosis is difficult to tell you without all of his information. He clearly has coronary disease, which is manifest by the plaque in the artery. His diabetes will make him at higher risk for events also. There are basically 2 things that can happen from plaque. First is rupture of the plaque causing an acute infarction or unstable angina, and the second is limitation of flow from plaque causing anginal pain. The prevention of the plaque rupture, really relies on optimizing all of his medications to control his risk factors and maintaingin a healthy lifestyle that incorporates weight control, exercise and dietary discretion.
The second, if plaque is severe enough to cause symptoms can be sometimes controlled by stents or medication. Usually blockages are intervened on if there is evidence of ischemia in the areas of a severe blockage. We generally use 70% as the level of blockage to cause blood limitation.