It sounds like a fungal infection, my partner gets surfers fungus and I treat it with 50/50 solution Lugols iodine, (you could use povodine iodine, but I find Lugols more effective), applied topically to the whole surface area of the rash, this has worked in erradicating this fungus. I also soak his wet suit in a 1/10 Iodine solution to kill any spores.
Ozone also works very well for small areas, but I haven't used it for large areas.
You do not want to create a moist environment, so I would avoid any really oily treatments, because he will still sweat underneath the coating and this will provide nice growing conditions for the fungus.
Get as much sunlight as possible, on as much of the rash as possible, (if you can get him to nude sun bathe for 30mins at a time, this will also help.