There are a lot of things going on here...
The hair loss could indicate some problems like parasite infestation, inflammation or malnutrition. Without physically examining your cat we won't be able to tell you what exactly it is, but you should keep in mind that parasites like mites (which can be microscopic) can spread over the whole skin and some can also infect humans.
Bad smell almost always indicates a fungal or bacterial infection. This could be the main cause, but it could just as well be a secondary infection in addition to the main cause.
You write that you used Sebbaderm shampoo on him (and I assume "F10" means F10 Germicidal Treatment Shampoo). That was a good idea in case some substance in his fur irritated his skin. Since this shampoo is designed for cats, it at least cannot do more harm. Washing cats with shampoo designed for humans can irritate the skin even more because human skin (and shampoo) is more acidic than cat and dog skin.
I'm not so sure if it was a good idea to start him on antibiotics without consulting a vet first. I strongly hope the antibiotics are formulated and dosed for cats. Please never give any cat any human medication without consulting a vet first, you might poison your cat.
Antibiotics do have negative side effects, especially on the beneficial bacteria in the digestive tract. He may get diarrhea or bloating and lose his appetite in the next days due to the antibiotics. But if his problem wasn't caused by bacteria, antibiotics cannot help at all.
So, after treating him with both antimicrobial shampoo and antibiotics, it's rather safe to assume that he doesn't have a bacterial infection. This is important information that you need to tell the vet to steer the treatment into the right direction.
The only thing I can advice (besides going to a vet) is to carefully rub a little bit of castor or olive oil on the bare skin. The oil stops the skin from drying out and becoming even more itchy and it separates the scab from the skin, reducing the risk of injury when the scabs are scratched away.