why does his parent give him a nickname "BOO BOO"?

Correct me if im wrong something about him being a little kid and he used to suck on his fingers または something

whenhe was younger he use to suck on his fingers and his parents 発言しました it made him have a boo boo face and it just stuck

When he was a baby he would suck both of him thumbs and make a "boo boo" face, so his mom started calling him that and it has stuck.