In the movie Goosebumps, every monsters got sucked into the book, including Hannah, but in the latter part of the climax, the invisible boy is "shown" typing on a type writer. Wasn't he also supposed to be sucked in and never come back, like the others?
Stine had to stop writting when Slappy crushed his fingers(he was naming the creatures one by one... I bet to avoid technicalities that could lead to a character roaming free and also to ensure that Hannah would be out of the list, he thinks of her as his daughter after all) But then he gave the typewriter to Zack as he got swallowed by The Blob that ate everything (which acted a bit too much like Monster Blood).Zack then wrote "all the monsters". Clearly he is not familiar with all the anthology- the twist about Brent Green was very likely unknown for him- and he was thinking of ending things as fast as possible.The invisible boy, if it is the one from My best friend is invisible, is not a monster. He is just invisible.
In Goosebumps, you can't create a book that would instantly make the monsters get sucked into the book without writing about each of the monsters. R.L. Stine had to remember and write every character/monster he created, and then end the story in such a way that they all get sucked into the book.
What we can assume is that Stine just forgot about The Invisible Boy, and hence, he is still outside the book.
People are saying he is still there due to him not being a monster but what about Hannah and Slappy they are not monsters just a girl and a puppet well a talking puppet but you get the point but the reason I think he is still in the real world is beacause he was a normal kid effected by the books some how thus making not a monster or a figment of r.l Stine's imagination just an invisible boy but maybe I'm over thinking this to much