Hi there and thanks for your question!
While I can't say for sure without seeing your cat what is going on I can tell you that I have seen several cats with hair loss like this. We usually attribute this to overgrooming due to stress. It's often hard to tell what is causing stress in the life of a cat, but it can be anything from new company coming to the house to another cat in the house to something silly like you moved his favourite chair to another room. Often there is no discernable cause (and it is these cases where I have owners upset at me for suggesting their cat is stressed!)
If there are no lesions on his skin then one option is to not worry about it and do nothing.
However, many cats respond well to a medication called amitryptilline which can be prescribed by your vet.
You may also want to look into a product called feliway which produces "happy cat" pheromones in the air and really helps to reduce stress levels.
It's not a bad idea to have your vet check a urine sample on him just to be sure. There is one theory out there that some cats will have stress-related overgrooming due to bladder pain.
I hope this information helps! If you have more questions regarding this problem, just hit reply. If I am online I will respond right away. Otherwise I will respond as soon as I come online again. Or, if my answer is sufficient, then please hit accept. Thanks!
Dr. Marie.
The above is given for information only. Although I am a licensed veterinarian, I cannot legally prescribe medicines or diagnose your pet's condition without performing a physical exam. If you have concerns about your pet I would highly advise contacting your regular veterinarian.
Veterinarian
Compassionate licensed veterinarian with >20 years experience with cats, dogs and pocket pets