See as he's not too lame or it's just a "bump" it might just be a simple splint instead of a fracture. You can often see splints in younger horses as their spint bone isn't fully connected to the cannon bone yet (there starts off with a ligament there and that ligament turns into bone over time) and don't know where they are putting hooves and often can "bang" themsevles while in training.
Usually not seen in older horses (over 5) because the splint bone is fully connected to the cannon bone but can happen.
An upper splint can cause severe strain of the ligament running down the inside of the leg. This can result in inflammation and healing of the injury by ossification (other wise a boney bump). It's usually only the bad lower ones that actually fracture.
The reason I suggested x ray was because actual fractures of the splint bone can be bad because there is always the risk that until full healed, the piece of bone above the fracture line could move and interfere with other important ligaments. Especially that close to the hock. But once again it's usually only the bad lower ones this is assumed with because of the higher risk of fracture.
I think in your case it's probably just a high splint and not a fracture given the history. Which is not a big deal as long as there isn't too much stress put on that inside ligament from the boney bump that heals in place. Which from your explainations doesn't sound like there is.
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