I noticed my horse trying to rub his tail on walls, fences, etc. the last day or two. Upon closer inspection he has a flaky/scabby patch about the size of a nickel and he's rubbed out the hair on/around that spot. I know that's a common place for sweet itch, but I'm curious if there's anything else that could be causing it? He gets horrible itchy welts from insect bites every year, but this is new for us.

Ticks at the bottom of the tail bone

They don't seem to rub down there when presemt, but will at the top

Ticks or pin worms. Pin worms are extremely hard to get rid of. They are in the rectum and feed off the mucus. Don't really harm the horse. But cause them to itch because they lay their eggs around the anus. 

My mare does this to, same exact spot every spring! We had an extra long wet season this year so its just now finally starting to slow down! LOL I wormed for pin worms and washed around her lady parts with a shampoo my vet suggested. ALSO......... she had a pretty good sized " Bean" in her vagina that I took out. That gave her relief more than anything. But I do keep her on Kinetic vets " Skin and Allergy" formula and I see a difference with her itching and breathing. 

What shampoo do you use for that? 

Our gelding does this when he needs his sheath cleaned.

Its a flea/tick/ dog shampoo. Can't remember the name off hand.