I think he's lacto-ovo vegetarian, but I couldn't say for sure. The reason he wears leather is simple: he (like the rest of us) knows that many people eat meat. With all those slaughtered cows, either one makes good use of their skin (by making leather) or it rots. If an animal has to give its life, then some good should come of it; it shouldn't all be for nought.
I would say it's highly unlikely he's a vegan, since vegans tend to avoid animal products altogether, including leather. (Plus, we know he likes chocolate -- there's loads of milk in that!)