Board Thread:Linguistics/@comment-1595565-20150508010307/@comment-28303871-20150508012741

Sadly, you cannot actually copyright a language itself. You can however copyright any documents you create describing or using the language.

I wouldn't be particularly concerned about doing so, though, unless you expect his story to achieve sufficient popularity to gain a cult following similar to Star Trek's Klingon or Game of Thrones' Dothraki, in which case some people may want to claim your work as their own. Generally, I don't think people see conlangs and think, "I should claim that as mine!" It's up to you though, of course.