I sympathise, that is just the sort of thing that my mare does occasionally, for no reason at all sometimes. She's TB and is still not very grown up at 20 years old, I don't know whether WBs mature a bit quicker than that!
Oh my goodness, how absolutely horrible, well done all of you, especially the poor horse, I hope he will have a full recovery. Like Walrus, I would feed high fibre cubes soaked.
Ive used sulphur, mixed with either pig oil or sudocrem, for mud fever, with great success, it worked after 2 years of low grade mf which had defied all my efforts and driven me nuts!! You can buy it on Ebay, its a powder, search for Flowers of Sulphur.
I used to have a Pyracantha in my garden, I have never heard of it being poisonous and it is very woody with evil sharp thorns, I doubt if any animal would eat it!