I am NOT a veterinarian, so take my advice with a spoonful of salt.
What I would do in this situation is try to shave the area (electric hair/beard cutters work fairly well in my experience, just be careful about angling it because a cats skin is thinner than a humans).
I would then clean it using clean water, unless the area looks like it might be infected in which case I'd use medical wound cleaning solution.
If the area looks clearly infected or smells I would immediately go to the vet, no matter the specialisation as the cat will need medical assistance and probably antibiotics.
I'd then cover the area with medical grade cotton and medical tape, or bandaids (The dressing might not work, depending on how tolerant the cat is.), depending on what's available, inspect and change the dressing daily (more often if keeping him indoors is not possible).
And try to keep the cat indoor until it looks healed (especially if the cat does not accept the dressing).