Two months is really too long for a horse to be off from popping a splint. I think you have something else going on. The ones that do get lame, usually get over it in about a month. When the splint hardens, then it also is not sore to the touch anymore.
If the bump has a lot of hot swelling and is really sore to the touch, I would consider the possibility of a fracture of the splint bone. They usually do not resolve without removing the broken off end of the splint bone. If the bump is mid-canon, it would be the very tip of the tiny splint bone.
Or, this horse could be shod so far from level that he continually interferes by hitting the bump with the shoe on his opposite front hoof, keeping it sore. This is very common if the horse is not trimmed 'level' before being shod. Some owners and some farriers try to make a horse appear straighter legged when they actually toe out and need to be trimmed and shod that way. When you try to make a horse look like he is straight when he isn't, you can cause chronic interference and problems like you are seeing.
You can also see this if some horses get too long footed between trims or shoeings.
Either way, I think you have something else going on.