I can suggest a couple of ways.
(1) ask him to draw a clock showing 3 o'clock. Alzheimer's patients lose their spatial sense early in the disease and cannot do this correctly. If you do it every few months you will likely see a progressive deterioration. The other issue to track is his competency. See my recent blog at: http://drlifestylesevidenceforhealth.Blogspot.Com/2012_02_01_archive.Html.