Hard bumps can actually be caused by blunt trauma, such as being kicked by another horse, it all depends on where the kick landed, and how hard it was ect. Splints are caused on the legs when a horse kicks itself while moving, and it is easily diagnosed because it is a hard lump on the lower leg. How big is it? Does it look big enough to be caused by getting kicked? He could have also bumped his shoulder into something playing with neighbors, or running in a pasture or turn out or something. If it is relatively small, it could be a horse fly bite. But generally after a few days those go down in size. If it hasn't gotten any smaller, and has possibly even gotten bigger in the last few weeks, I'd have the vet come out and look at it again. It might be a cyst or something that needs to be lanced, but generally those types of things are soft, not hard. Hopefully you at least get rid of it, even if you never figure out what caused it.