What Texas said. The horse will always have an ugly scar on his leg. It will diminish some with time, but will never go away completely.
My TB has a similar scar, interestingly enough on that same back leg. It's unsightly, but hasn't affected his soundness. He must have been injured as a young foal, because even his racing trainer didn't know how he got it.
As long as he's sound on the leg, it's just a cosmetic flaw.