Piñata is the Spanish word (I believe) for a container that is decorated, and filled with small toys or candy (usually), that they hang up and break open with sticks at parties or celebrations for fun. For that statement however it means he's gotten used to being beaten on or pushed around, either literally or emotionally depending on the context.

I agree with Alice. He's being (figuratively) treated like a piñata, and now finds it normal: as if he's being hung up and beaten with sticks until candy falls out of him. Nice imagery, isn't it? :) http://www.123rf.com/photo_5619364_boy-swings-a-stick-at-a-pinata-at-kid-s-birthday-party.html

Alice already answered, but I would like to add another similar phrase used figuratively like this is "punching bag". "He had gotten used to being her punching bag"