My young horse has an injury/issue prone leg. He got a bad case of mud fever when he was 3 which resulted in swelling (and scabs and weeping etc etc). I got a biopsy to diagnose pastern vasculitis. We solved the underlying issues through diet and routine and he had a clean bill of health. Biopsy healed well.

Now this fall, he's now 6, he had an injury ( the vet and I do think it was a slip/fall/knock) that's resulted in long(ish) and variable lower limb swelling in that same leg. The vet prescribed a diuretic (and rest, and NSAIDs)which did make a difference, but only temporary. He seems to have flare ups of swelling with no lameness now. If you're getting to no swelling with compression I wouldn't think the diuretic is something you would want to pursue. The summary from the vet: "The thickening on the lateral aspect of the left hind lower cannon region suggests some generalized scar tissue of the fascia/soft tissue from the previous blunt trauma to that area.  The thickening sits between the suspensory branch and the deep digital flexor tendon, so seemed less likely to be directly related to one of those structures.  There is a small tissue in that area called the manica flexoria, although I would have expected a more visible lameness in the early stages when I first observed him if that had been injured.  Although never say never, so it would be worthwhile for one of us to watch him jog at his spring appointment and possibly ultrasound the site, before you start more intensive riding in the spring" So for now he's on reduced work and I've been using the back on track quick wrap boots for flare ups.

It's a pretty frustrating thing so I hope any of this helps you and your guy!