He has to show damage. The other two main elements of slander (falsehood and insufficient attempt to prove the initial statement was true) have already been met.
Since Mr. Holmes is not a public figure (or was not before today), the standard to prove slander is much lower than it would be for a public figure (no ‘actual malice’ test applies here from what I have read).
It essentially would depend on what a jury believes a man’s reputation and honor is worth and how much they have been damaged. Juries tend to be overly generous.
What if someone took a shot at him, how much would that be worth?