I had one that used to get fluidy swellings by his sheath. A course of detox cleared it up, and he stopped getting it at all when I started doing regular spring/autumn detoxes. May be totally different to what your boy has but worth a try.
I’m looking for Lactobacillus (plantarum and others) and bifidobacterium longum. From what I’ve read, the colonising effect in horses is transient but does have benefit in rebalancing the hind gut biome. Trouble is, Microbz has a tonne of other stuff as well, and no idea in what quantities. Or...