Short Story - Next stage of human evolution has a language they all understand. Protagonist is father who can only read it

I read this somewhere in the early 1990s in a science fiction anthology, I think. The protagonist is a man with a young son. He learns that some of the books that his son has are written in language that he and his son can understand, but most people cannot. He eventually meets the people behind the books and it's explained that the books are written in a way such that only mutants can comprehend it, but that they all also had further talents. He's tested, and it's revealed that his only talent is being able to read the script. Near the end of the story, all of the mutants are going somewhere else (another planet?) and he's told that he cannot join them, but that they would provide a replacement for his son. It ends with him and his wife meeting the simulacrum and finding it indistinguishable from the real thing.

The only other details I recall about it were that the place where the books came from could only be seen by mutants as per the language bit.

Answer

Och. I found it by pursuing the threads of memory. I repeatedly read and reread Asimov's Young Mutants and Young Monsters books as a child. The former contains "The Children's Room" by Raymond F. Jones. I found a copy online (unsure of the provenance), which let me go through and find that it is the correct answer.

I can't find a summation of the plot online yet, but here is the IMDB summary of the "Tales of Tomorrow" adaptation:

A couple -- Bill and Rose -- are having increasing difficulties with their son Walt: he talks down to his parents like they are stupid and reads books written in a language that no one understands. The books come from a "children's room" that supposedly does not exist. Once the room is discovered its true meaning is revealed.

Using the aforementioned digital copy, I verified the other details. His son, Walt, is sick and asks his father, Bill, to return a book he got from "The Children's Room". Much to Bill's surprise, the librarian says that they have no children's room. However, he finds the room in the building and quickly learns that they're recruiting mutants for a conflict far in the future. And indeed, his mutation allows him to do nothing more than read their script, while Walt's is indispensable. And indeed, it ends with the copy showing up.