A partial occlusion still allows adequate blood flow to the brain. Operations are not even done on carotid blockages until they are 70% or greater. Peripheral vascular disease, high cholesterol, smoking can all contribute to occlusions. I suspect the blood supply to his brain is still being maintained by the partially occluded vessel, and no anastomosis or bypass vessels are present. Depending on his history, degree of occlusion, and other factors, he may need treatment to prevent further progression of the occlusion that could result in a stroke. Hope this answers your query. If you have further questions, I would be happy to answer them.