Your cat may have been bothered by something and pulled some of his fur out. Sometimes this can morph from a one-time problem into becoming a habit --which then becomes a behavioral problem.
One of the most common reasons for cats to pull their fur out is an adverse reaction to internal or external flea treatments.
Food allergies can also cause a cat to pull their fur out. Food allergies are very difficult to pinpoint. A recent change in diet can cause problems too.
Fleas can bite just once causing flea dermatitis and cats will pull their fur out around that area. Once the bite heals up, cats can sometimes get into the habit of pulling their fur out.
Or your cat may just be in the habit of pulling his fur out because of some type of stressor.
If the case is that your cat is in the habit of pulling his fur out, you need to re-train your cat not to pull out his fur. Can be challenging, but is doable.
I would suggest that you buy a wrap-around cat harness or a Thundershirt that will cover most of the cat's body. This will immediately prevent him from pulling his fur out.
If it's the back or the tail where he is pulling out his fur, there is also a product called SELF-STICK WIDE BANDAGE WRAP that you can wrap around the cat's body which will also stop the cat from pulling his fur out. Make sure that you don't wrap it too tightly.
I suggest that you supervise your cat closely while he is wearing either the harness or Thundershirt and/or if you decide to use self-stick bandage wrap on your cat.
Next, look into changing the type of litter you are using. Choose a litter that is non-scented. Gradually add a cup or two daily of the new brand of litter to the old brand of litter so that the cat doesn't notice the change. Eventually, swap out the entire old litter and use the new litter exclusively.
Buy a Feliway plug in diffuser and plug it in where your cat hangs out. Feliway is a synthetic cat pheromone that calms anxiety in cats. Feliway takes a few days to work so don't give up on it if you don't see results right away.
Prednisone will help temporarily. But you still may need to interfere with the cats HABIT of pulling the fur out so you may still need to immediately prevent the cat from accessing the area's of his body where he is prone to pull fur out.
Increase the cats play time. Buy him a TURBO SCRATCHER ($12 Walmart) so he can play independently. Flick a feather at him so he jumps up and down. Increased playtime will get his mind off of pulling his fur out.
I've read that some people who had cats that pulled their fur out -- rubbed orange zest in their hands and applied some of it to the cats coat where he was biting at himself. The cat didn't like the taste of the orange zest and stopped grooming himself (temporarily.) By the time the orange zest wore off, the cat had forgotten about pulling his fur out. (I can't recommend this remedy as I've never tried it on any of my own cats so I don't know if this would be successful. I'd also be concerned about the orange zest getting into the eyes of the cat.)
Hope these suggestions are helpful. 