At this point, without better photos, I'm pretty confident that he's a very dark grulla. They aren't particularly common but they do happen. Jaz Poco Sterling - hammer horses
He's not quite as dark as your boy, but it gives you an idea of how dark they can be. Not all grullas look like the ones that PBR posted. In fact, those examples are especially light with very dark, prominent dun factor that isn't always present. Not all grullas have frosting in the mane like that and all most definitely don't have that awesome of dun factor on the ears!
I know I've seen other grullas as dark as the OP gelding. Unfortunately I don't have any saved, but I think I've found them on the online BLM adoption site. That seems to be the best place to find awesome expressions of dun