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If your dog had an injury or sore there, there might now be bacteria present and it's causing a "hot spot." The best way to treat a hot spot is to shave the area completely and wash it well with an antibacterial agent. Then apply neosporin or other such ointment to the area. Keeping the area clean and dry will help to speed the healing.
Because they have so much hair, you will also want to think about having a professional groomer trim him in the spring. This will help to lessen the chances of hot spots.
Let me know if you still have questions.
The only thing that I would do differently is wash it with a peroxide wash (1 part peroxide to 4 parts water ) or a betadine wash (1 part betadine to 4 parts water). Then dry it REALLY well and THEN apply the neosporin.
Some hot spots need vet treatment so, if you don't start to see improvement, you will want to see the vet.