That's one of the best uses he can make of his class - learning the different forms, clarifying the way something is expressed by the anime character as opposed to how it would be expressed in different "real life situations" etc. And with Japanese especially, there are a lot of different forms of everything.
Listening to the language - whatever he likes and is interested enough in to pay attention to and enjoy, is essential to making some things happen with neural pathways.
If he were a little kid, growing up in Japan, he would hear plenty of squeakyboy stuff, as well as the "regular" stuff from mom and dad.
This is one of the advantages of getting actual TV stations, if he can. On the news, he can hear one kind of speech, and while he jogs or showers or whatever he does, he can listen to pop music (if pop is the music he likes most). Liking it makes the neural pathways do what they do better and more efficiently