Is your boss a whiskey drinker? I ask because there's whiskey, then there's whiskey, then there's whiskey. Then there's even whisky.
Is he a bourbon drinker? Rye? Scotch?
If Scotch, you could buy him a $75 bottle of Lagavulin (an excellent Islay Scotch), but if he only likes Highland Scotch, it'll be a wasted $75 and you would have been better off buying him a $20 of cheap Highland Scotch.
Not trying to be a stick in the mud, but I'd hate to see you throw out a Benjamin and have your boss say, "well, it's the thought that counts". If you have some idea of what he likes, let us know and we'll come up with suggestions.