This is called feline Alopecia. Yoou can read about it here:http://www.thepetprofessor.com/articles/article.aspx?id=352
Sometimes it is caused by allergies to the grains that are used as fillers in many commercial cat foods. It can also be a result of stress. It is not likely to be an excess of cortisone but can be hormonal if your cat is diabetic or has hyperthyroid.
Look into changing his food to one with no grains and if he is stressed try a natural product like composure liquid at www.onlynaturalpetstore.com or rescue remedy or feliway available at most pet supplies.
Please let me know if you have any more questions. I will be happy to help in any way possible.