my cocker spaniel skin care

Question
my dog has a dark dry spot under his mouth down to his chest  which all his hair has fallen out and has an un- pleasant odor what to do and what do you think it is?

Answer
This sounds like a massive "hot spot" and it sounds like it's gone far enough that he's going to need an antibiotic.
Please go to the vet right away and get this properly diagnosed and, if possible, get a referral to a Dermatologist.
Have you been bathing him and treating this with anything?
Your vet may want to prescribe a steroid (like prednisone) - ask if it's possible to treat it topically.  Much safer and easier on your dog.
Delores