I should answer with fake badass things that make no sense like 666 ON HIS SHOULDER BLADES BECAUSE HE'S SO HXC LULZ
I COULD ONLY FIND THREE. I GIVE UP. Although I actually probably could see him getting sleeves if tattoos were actually his thing, though they probably would be more patterned and tribal and abstract than representative of anything.
1. Upper arm. Because. Um. He is a wolf? Yes. Lulz.
2. Same spot, other arm. (The middle row, far left) Moral compass. Everything is black and white, and all that.
3. "Whoever fights monsters should see to it that in the process he does not become a monster. And when you look long into an abyss, the abyss also looks into you." Between the shoulderblades. It's a long quote, so I imagine it would hurt, but I think he could handle it.