Thank you all for the suggestions! I will definitely give them all a shot.
@Demerara_Stables Great idea with the piece of hose, I have an old one I can cut up.
He is fine with fly spray.
Yesterday we practiced just existing within 10 ft of a slowly dripping hose, which still was an issue but we ended on an ok note. He is a super weird horse to train because 99% of the time it feels like trying to train a brick wall, I end on some kind of not horrible note…and then the next day he comes out and does it PERFECTLY. So my hope is that if I keep working on it every day he will just walk out one day ready for his bath .
I unfortunately do not have hot water at the barn but I would love to! However, it’s an uninsulated pole barn so I am not sure if that would work in the winter. But, the barn/shop is somewhat attached to the house (…we did not build it, but it’s basically a large strange pole barn complex), so I have thought about maybe running hot water from the house into the mudroom that attaches the two, and I could run a hose out from there. My TB would certainly appreciate it, and I soak their food so having hot water out there would be great.