Maybe he is losing hair due to stress.
I would certainly get a blood panel run on him to determine whether it could be stress or physically related; fungus, bacterial, etc.
What kind of sprays do you use on him? It could be a result of allergies; my mom's old arab will lose hair if you put anything oil based on him.
I would try something like Aloe Heal, or some other form of gentle skin moisturizer before going to something like MTG...no offense to those who will suggest it, I just don't think it is that gentle to alot of horse's skin; I mean they tell you to apply it with gloves, and a horse's skin can be alot more sensitive than ours...soooo...
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."