I'm sorry to hear that Thor is having some skin issues. A focalized skin lesion can be caused by a variety of reasons, such as trauma/irritation, allergies, or a skin infection (bacterial or fungal). If the area is warm to the touch, has a moist discharge, doesn't improve or Thor seems itchy/uncomfortable, I would recommend he be evaluated by his regular veterinarian to determine the underlying cause for the skin issue and start any treatment that may be needed.