This is a standard pose for Koreans. :P
He is one of my many favorites... well, they are all my favorites!
He is in the youngest group at the school.
According to Korean age, he is 5 years old, which means he will be 4 sometime later this year.
When you are born, you are 1. After that, you add one year every New Year (Lunar New Year, generally). So, between the Lunar New Year and your birthday, it is a 2 year difference. Between your birthday and the following New Year, there is a one year difference. That means, if you are born in December, within a month or two, you are considered 2 years old.