Just as a note, depending on the disposition of the child, I would be a little careful with Fullmetal Alchemist. It's quite a bit more violent, bloody and horrifying relative to the other things in the thread. I'd also say the same of Kimetsu no Yaiba. There are some quite violent and upsetting scenes in both that a nervous or easily spooked child might react poorly to. They are both excellent shows but you might want to wait until he's a little older before showing him them.
I would also second Mob Psycho 100, it's a really good show.
One Piece and Naruto are fine, Dragonball is fine.
Tengen Toppa Gurren Lagann would also be a good get. Big mechs are cool and Trigger really killed it in the design and sense of scale of the show.