Have you considered a wither pad with a traditional cut out to relieve pressure on his withers? I am currently in the same boat with my mare and am looking at purchasing one of these pads, Equine Comfort Half Pad with Wither Relief
. This pad is a little expensive but I have seen similar pads for much cheaper ($50.00). Just food for thought :)
Oh and I absolutely love your horses mane! The colouring is beautiful.