This sounds exactly like myself. I was multilingual very early as well, and took French, Spanish, German, and Latin concurrently in school. I also enjoy creating human languages (conlanging). My main bit of advice is that he join the Language Creation Society (conlang.org).
The only conlanger I know of who makes a decent living creating languages is David J Peterson. He created the GoT languages, the grounder language in The 100, the various languages of Defiance, and the Dark Elf language in Thor the dark world among others. As this is the most financially successful conlanger whom I know, I am not too certain how much call there really is for language construction. Still, joing the Language Creation Society will at least put him in the running the next time there's a contest to create a language for HBO.
EDIT: I keep building languages as a hobby, and learning human languages when time allows, but ultimately as I grew older it became necessary to pour my efforts into my other love: technology. Life is expensive.