Years ago my parents rented this anime from the local video store, I was very young, but I remember a few details. There was this boy from the real world who I think went to sleep and was carried away to this city in the clouds, (in his dreams?) He had dark hair and was cowardly. He met this girl on a train traveling through the clouds. They carried him back to the castle and he met her father who looked like santa clause. He gave him the key to the city. But there was this dark goo the king had sealed under the castle, trying to get out. His court jester was evil and tricked the boy into releasing the dark matter which ate the king. And covered the cloud city in blackness. The boy challenged the beast, and used this phrase something about pajamas? (the boy was wearing the PJ's through the show) that's all I remember. I don't know the name of the movie but it's a movie not a TV series. I would really appreciate help.

Little Nemo
Adventure in Slumberland

Thank you so much!