I think that it may be a bite would abscess. It can look like a large pimple that has popped. If he is an outdoor cat and it is a wound of unknown origin your veterinarian will probably want to booster his rabies vaccine. Your vet will also flush the wound with a surgical scrub solution, give your cat an injectable antibiotic, and send you home with antibiotics by mouth for 10 days to two weeks. If it is a very serious would or your cat has stopped eating or drinking due to the infection your veterinarian may want to perform blood work on your cat, and give him IV or sub Q fluids.
If it is not a bite wound abscess your veterinarian may want to do a biopsy of the tissue for a definitive diagnosis.
Hope that helps! Thanks, Dr. Cheng
Thank you. You were exactly right about my cat's head! He's doing great now.