Kaibutsu-kun: Kaibutsu Land e no Shoutai (1981)

Its spring break and everyone is going on vacation but Heroshi doesn't have any place to go but when Kaibutsu-kun receives a letter from his father saying that his father is sick he asks Heroshi to come with him to monster world, over there Heroshi turns into stone after a look on the king of that world but Heroshi and his friends do whatever they can to save him.