What age breed and weight is he?
how long has he had this?
is there any chance he got a substance on his tail or a burn or other injury?
He may have been bitten by one of his siblings or it may be a birthmark
It now can a hot spot, pyoderma or impetigo Please compare to this link:
Immediately give him one to two mg per lb of his weight of benedryl to reduce the swelling. Try the lower dose first.Shave or trim all the hair around the hot spot Then apply neosporen.That will arrest the bacterial growth.
You will need to treat it every two hours for few days to get rid of it - I know it is a PITA but then it will be gone.
It is best treated with oral and topic antibiotics and sometimes steroids.However, sometimes this can be treated with steroids but there are side effects of Pred..In addition, steroids weaken the immune system and can damage the kidneys.
The bacteria in his mouth will impede healing - so try to deter any licking. Exposure the air whenever possible will be very helpful
You can use oatmeal baths to soothe your baby
Give him a daily vitamin suppliment with omega 3 fatty acids.
As yeast infection is possible. Please look at these two links:
"Shampoos: While degreasing shampoos such as the benzoyl peroxide (oxydex®,pyoben®) and sulfur/salicylate (sebolyte®, sebolux®) shampoos will help remove the skin oils feeding the yeast, there are shampoos that are specifically anti-yeast. We prefer the 4% Chlorhexidine shampoo called Chlorhexiderm Max as it both strips skin oil and kills yeast; however, other anti-yeast products include Selsun Blue, Miconazole shampoo, Nizoral shampoo, and more. The pet must be bathed twice a week to start and the shampoo requires a 15 minute contact time (meaning do not rinse the lather for 15 minutes). "
Try the above treatment and see if it resolves this.
I hope it does and your baby (and you) get relief very soon.
I will always be here for both of you.
Have a wonderful Thanksgiving,
Sincerest best wishes,
Here is info on Lymes:
That illness presents with a high fever and sore swollen joints.
Pleas let me know how your baby is doing.