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Most of what you have heard are about the active ingredient causing neurological problems. Or using a dog product on a cat. These are mostly the OTC products.
Just a bit of education. I'll get off the soap box now.
As for your cat. You say the skin does not seem red, tender. or crusty. Does it feel thicker than other skin? Feel an area on his back away from the bald spot.
Is he and insidecat outside cat or a combination of both?
If the skin is not inflamed, his eating habits and demeanor have not changed, we can probably rule out an infectious or toxic cause.
We are probable dealing with something in the environment. I have seen this with cat (and dogs) going under cars fences and sheds. They catch the hair as they pass under what ever is the want to get through. The only way to tell is to look for hair still caught on the offending object
The one other option and it still doesn't fit is a skin tumor seen in older cats. This usually occurs in the area you mention. The cats develop bald spots and the skin becomes moist red and tender. They develop bumpy tumors under the skin. The thought is that it is a reaction to vaccination being given all in the same area. This is why most vets will now separate the sites where the vaccine in injects.
If you notice any of these symptoms. Please get him to the local vet right away.
I hope I was of some help.
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