Is your dog regularly treated for fleas?
Has he had any diarrhoea recently?
Thanks for the extra info.
The first thing that springs to mind when you describe the signs was the fleas - some dogs can be very sensitive to flea bites, and so the probelm does not always occur in all of the dogs in a household. If a few flea bites cause irritation, then this can start a viscious cycle where they continue to chew even after the fleas have gone. Sometimes you need to be able to break this cycle before you can really get a resolution to the problem. This may require medical treatment in the form of anti-histamines or steroids, or the use of a buster or elizabethen collar just to give the skin a break and allow it to start to recover.
Sometimes if a dog has chewed persistently at an area they can cause a skin infection - they carry loads of bacteria in their mouths and when they lick and chew they push this into the superficial layers of the skin and cause a dermatitis. This can be damp and can be a bit stinky too. If this is the case, then antibiotics are used and also a mechanism to reduce the chewing.
The other thing that I would definitely do would be to get your dog's anal glands checked. These are small scent glands that sit just inside the anus. They are emptied normally by the action of the faeces passing them squeezing them, giving the faeces it's nasty scent. If the dog has had a period of loose stools, the glands can become full and impacted and become uncomfortable. The dog then tries to chew them to get relief, but they can't get to them so they chew the tail base instead. If the dog is gradually producing little drops of anal gland fluid as he chews, you will get a constant odour that is instantly recognisable if you have smelt it before!
I hope this helps,
You would need to get antibiotics prescribed by your vet - you cannot aquire them without a prescription. Your vet will be able to discuss the directions for dosing with the antibiotic, but I would also recommend a collar for the dog to stop chewing and give the antibiotics chance to work.
It is difficult to know what these marks are without seeing them. They may be normal areas of pigmented skin, or they could be areas that have been inflamed over a long period - chronically inflamed skin can often appear this way. Usually areas of infection are scabby and red or damp looking.
I hope this helps,