Tail rubbing.

My Mini has begun rubbing his tail out! I have owned this horse for 16 years and he had never done this before until last summer. Below is a list of things I have ruled out and tried, I'm out of ideas, if you can think of something else PLEASE HELP!

Has never missed a worming in 16 years. Wormed on a rotation.
Haven't changed hay supplier in 3 years
I use minimal fly spray, because one of my other horses is allergic, when the rubbing started I tried using a lot more on my Mini, especially his tail.
Also tried not using ANY fly spray, no difference.
Rubbing discontinued briefly during the fall, started again in January. So I don't think its bugs bugging him.
Tried conditioners, shampoos & lotions
Anti-fungal treatment
Dex shot and cream (AKA Benadryl cream)
No bedding to be allergic to, he's always turned out.
Supplements and grain have been the same for 5 years.

I should also add, last August he had some weird allergic reaction where he swelled up everywhere, especially his sheath, that's when the tail rubbing started. My vet, said allergy triggers in horses are nearly impossible to find. He gave my Mini a couple doses of Dex. And the hives and swelling cleared up with no return, but the tail rubbing continues. Could it just be a habit now? He used to have a beautiful little tail, but now his dock is hairless. Can any one here see what I may have missed?

Sometimes horses rub when their nether region needs cleaned. When was the last time his sheath was cleaned?

Oops sorry didn't see that last paragraph

My old mustang did that all the time because of allergies. My vet put him on medication

I'm going to try this again
Vida gets very dry flaky tail bone skin. I scrape as much of the flake off then drench it with MTG. It seems to help her when she's in tail rubbing mode, as well as keeping her nethers clean

I have heard that tail rubbing can become a habit, not sure if it is true or not though.

You say that the tail rubbing started after some sort of an allergic reaction and his sheath, especially, got swollen. You didn't say if it has been cleaned or not, if not, it could be that it needs to be cleaned. I wonder if it could be allergies of some sort, he has developed some kind of sensitivity to something since last summer. Have you talked to your vet about feeding antihistamines? They can be easily added to the daily feed.

My first thought was that he likely just needed his sheath cleaned.

My gelding sweats a lot around the general area, like from his flank, down to the back of his legs. Sweat always ends up itchy at some point, so I always make sure to keep him hosed of sweat in that area. Is this an issue with him?

Yeah, I forgot to specifically mention that yes I have cleaned his sheath repeatedly:(

MTG was one of the products I have tried.

I'm hoping I don't have to start oral antihistamines with him, the antihistamine creams aren't doing anything, all of his other allergy symptoms have gone away, but if he keeps up rubbing, I probably will try it.

Some type of mites? Mange?

Have you had your vet do a skin scrape and look at it? Biopsy?

Very weird...Could just be a naughty habit he can't break.