not an idea what has cased it, usually dry skin is an alergic reaction to something, what in his enviromnent has changed lately?? (rember it could be soething you yourself changed like fabric softenr or dishwashing detergent.
mabe take him to the vet just to be sure.
however there is a wonderfull product called BagBalm vailable, most farm supply, some petstores and such carry it its in a green tin and was developed by farmers for cows udders as they got very dry and chapped after milking.
its like a grease but its a WONDERFULL moisturizer for extreemly dry flaky chapped skin. and you yourself can use it (good for chapped feet knees elbows ect.
just a little bit massaged into the dry areas you should see improvement in a few days.