The 'tanking spreadsheet' is only a guide on stat distribution. How much damage you take in a raid also depend on how good you are at using cooldown. If the mox parse says he's taking less damage than you, that means he's more skilled at anticipating damage. You should take some tanking lesson from him.
On the other hand, if he used his superior knowledge of fight mechanics and cooldown rotation along with your guidelines as to itemisation he would be a better tank that he is now. Noones perfect and knows everything, if he refuses to accept general knowledge to improve on his game then I feel sorry for the rest of your guild who has to pick up his slack.