As far as I know, there are no rules for this, but it's a really cool idea!
Personally, I would make it so that he has the tattoos carved into his body all over (1d4 damage, plus any disadvantage to Interpersonal skills the keeper desires as the character now could be considered deformed). Next, I would say he now has 1d4 points of armor (so each time they get hit, roll 1d4 to see how much damage is reflected), however, the pain nd suffering is not blocked completely, but instead is channeled to the mind. For each 2 pints of damage reflected they take 1 pint of sanity damage (round up).
I might actually use this idea in my games in the future, very creative. This is just how I would go about it, you could always homebrew however best works for your party. You could also increase the damage reflection indefinitely as the damage is really still taken as SAN damage.