Because of the two layers of hair chessies can get several types of skin infections, which I am sure you know. I shave mine in the spring.

put a few splashes of vinegar in a 5 gallon bucket, a couple squirts of "mane and tail" horse shampoo/conditioner then fill the rest of the way with water then splash it on him.

I would also stop brushing him so much, it is probably irratateing his hide.