Media: Children's book.
Cover: It had a 2D drawing of the kid and maybe some planets on the cover.
Year read: 2009-2012 (as a kid in kindergarten to second grade). Written sometime between 2000 and 2012.
Tone: Lighthearted, humorous.
Setting: Earth, alternate universes, other planets. In the universes/planets there were definitely alien characters with very long and strange names.
Characters: A boy who somehow moves between parallel universes and/or planets. An alien known as “Mr.” and then an unknown, very long, last name.
Language: It was in English and not translated, probably written by an American author.
Other: I think it might have been illustrated. It had multiple chapters.