I wouldn't worry about his caudal fin. It looks like a pretty straight split and those usually heal quickly on their own even without AQ salt, but it should help it heal faster. If the AQ salt treatments are working on his other fins, continue using it for no longer than 10 days. Since his fins are growing back and there are no other symptoms, I think he should be okay.
Like I said, they usually don't grow back the same as they were before, but to me, it looks like fin rot so you were already properly treating it with the AQ salt.