I might try him with a fungal shampoo! I’ve resorted to a rain rug currently so he’s not weighed down by a soaking fly rugMine keeps doing it this year, I've had him 10yrs and although his is sensitive don't normally have an issue, and haven't changed anything. Seems to be worse when it's extra hot and if we have a shower during the heat so not sure if it's humidity?
I have put him on antihistamine bought online which are helping, it almost cleared up until a small outbreak today.
However it's been wet agai so maybe that? I am starti g to wonder if there could be a fungal element