Actually, I don’t think it is for kids. Anime deals with very mature subjects. Even though it is illustrated, there are deep storyline arcs and character development that young children would have a hard time following. That’s why you see older teens and young adults watching it and reading manga, where it has its roots.
Besides, What other things would you advise your son not to do because it is only for kids? Play sports, take classes? hang out with friends? There are many things that we do as children, which carry over into our adolescent years, and then into adulthood. They just become more nuanced and deep.
I’m an adult who works full time and has raised my children to be successful college students. Yet, at the end of the day when my work is done and my house is clean, I play a video game (Genshin Impact). I love it. It is an escape and a stress release. Allow your son to have his.