i agree that it could be a flea allergy. Fleas over here have been horrible this year. My oldest STILL has fleas even after frontline & 2 days of capstar oral treatment! I can't wait for the frost! It could also be another allergy, from pollen or from anything that has changed in the house, like fabric softener, cleansers etc.
An oatmeal bath will help to soothe; try to get a hypoallergenic shampoo so that more problems aren't intoduced.
he could possibly have a flea allergy and even though he is on flea prevention he could have gotten bitten by just one flea(from outside etc.) and it set him off. your vet can provide you with a shot of demadex and it should clear it up be it a flea allergy or any other kind of allergy.if youre want to try an at home method to see if its just dry skin try a gental dog shampoo that contains aloe. hope he feels better:)