This condition, of microvascular disease in the brain -- is something that generally occurs over years.
Years of high blood pressure, high cholesterol, smoking, diabetes, etc -- all the risk factors for heart disease can also affect the arteries in the brain.
These arteries then start to harden, and affect small areas of the brain, creating "white matter changes" -- or changes to the brain itself, but often unnoticed because they are very small areas of the brain that are being affected.
However, after years, when these multiple small areas that are affected add up, you can get cognitive slowing, and symptoms that are similar to dementia.
This can be called "vascular dementia":
Read more about it here:
Unfortunately, because it involves the small arteries in the brain, there really is no treatment available for this -- just best to try and prevent it from worsening, by controlling blood pressure and the diabetes.
Of course, without knowing the actual results of the MRI, the above is simply my guess at what is going on, so speaking with his doctor/neurologist for a diagnosis is the best way to confirm this.