What is it that depresses you about the tattoo? Are you assuming that he went with OW to get it or something? Is it of her face?
Lots of people have tattoos on their backs. How about this - if it makes him happy (briefly, in the now), why is that a negative for you?
What if you complimented him on it? ("Hey - nice eagle/cross/symbol/whatever" - in kind of a bemused tone). Maybe ask him where he got it - say you've been thinking of getting one?
Just some thoughts to help adjust your perspective, especially in light of 25's observation that he may fear that he won't be forgiven.
