The scabby spots on his tail are not caused by scratching as they always start almost underneath the tail and the hair starts to fall out at the base where his tail bone brushes against his rump. It looks almost like it is caused from sweat or heat?
Are you sure it's not itchy? Even though you don't see him scratch his tail/butt, he may still be itching this area on a fencepost/tree, etc. when you are not around. I am asking about the itchiness because it sounds to me like sweet itch, an allergy that some horses get to the saliva of the female noseeum (biting gnat). My horse has this and the symptoms sound very similar to yours. It also sounds like he could have sweet itch in his ears as well. If this is the case, you could use Panalog (sp?) and rub it on the itchy area in his ears (the vet told me to do this for my "second" horse, Emblema, who gets sweet itch in his ears).
For addressing sweet itch on the tail/hind end, the following products have helped my horse so much that, as long as I keep applying them during sweet itch season, his symptoms dissappear and do not come back:
Toad Juice (the spray); Toad Stick (the waterproof stick); Endure fly spray (not sprayed but wiped on, against the direction of the tail hair and coat hair). These products also work well on the face, around the mane/neck and other areas (besides the inside of the ears) where sweet itch occurs.
Even if Arista's problem is not sweet itch, I still recommend The Toad Juice products for fighting general fungus and bacteria. I would talk to the vet over the phone at least, about his ears and see if he/she thinks Panalog will help because I think you have to get the Panalog from them.
I hope this helps.