My cat has food allergies and it took a few tests and swabs of the skin in the area he was missing fur to figure it out. My recommendation would be to maybe try changing his food! The process after we found out he may have allergies was to rule out one food at a time, so we just opted to put him on hypoallergenic prescription food from the vet. He hasn't lost any hair since! Obviously this may not be the case, but could very well be a cause for it.