Is he laying down more than normal? Is that why he’s getting the sores or is he just reopening existing sores? If he’s laying down more than normal, need to figure out why. Fetlock sores make me think sleep deprivation (horse falling asleep while standing, falling forward, and catching themselves) and a horse that is not comfortable laying down, but you mention he naps in his stall, so that’s why I’m wondering if he’s laying down more than normal. I’ve witnessed a few horses with foot pain that seem to lay down more than normal (in my opinion).
The longer he lays down, the more pressure he’ll be putting on those sores, the less likely they’ll heal (without help). I’d recommend in addition to the boots/wrapping/topical dressings, that you increase his bedding and make it softer. I think you’re going to need to do a few things to resolve it.