Does my dog have Dermatitis ?

Dermatitis is a general term meaning an irritation or infection of the skin. What you are describing could be one of several conditions including mange. Have your vet examine the dog before it continues to worsen.

GO fast- and be careful not to get him near you.

This could be a bunch of things, including ringworm which is actually a very contagious (to YOU) fungus. The things that cause this kind of problem are all different, and each one has a different treatment that won't work on the others. This time you do need the vet, in order for them to take a skin scraping and examine it to identify the problem.

If you have an e-collar, put it on him immediately to avoid any further dmage to his tail. The itching can be bad enough that they get into a cycle of chewing, and they will mutilate themselves in the process. You need to protect him from that quickly.

And the solution might be as simple as adding some oil (Canola or vegetable) to your dog's food if you feed him/her dry kibble. This was the genius solution suggested by the vet, which completely cured a similar conditon. Worth a try.

The vet is the person who will tell you about health or illness and treat as such.....................sounds to me like your dog is bored ,frustrated and trying to use use excess energy............ "He chases his tail a lot" =boredom, using energy, frustration, trying to entertain himself as h has nothing else to do...chewing relieves anxiety/stress and can become a habit.

I would have him checked to make sure is health is OK, he hasn't got any skin condition and increase his lead walking exercise so his chewing whether a skin condition or boredom doesn't become a bad habit he does..........................