The boil can be caused by an infected hair follicle, sebaceous cyst, dietary insufficiency or toxins building up in the circulatory system and parking themselves under the skin as a boil. treat with a course of blood cleansers to start. Continue this treatment for 12 weeks (a full blood cycle).
Dab castor oil over the boil daily to draw out the infection. You can cover with a light bandage. Alternate castor oil with colloidal silver on to the boil to treat the infection topically. It should burst and release its contents. This is a good thing. Keep applying the castor oil and colloidal silver until it completely clears.
I also recommend that you place your dog on a diet of raw meat and bones i.e. beef/mutton bones with meat on them or raw chicken necks and wings, mince, definitely no processed food and snacks. You can also add some freshly cooked vegetables and rice.
If your dog is not already on this type of diet, then I suggest that you introduce it slowly.
Also add 2 dessertspoons of crushed White millet and linseed to the diet once daily to improve tissue quality and repair.