??
I think Captain Tsubasa is the go to anime in that case, if you can leave the old looking graphics aside.
Maybe if you're a bit more on the cartoonish side of stuff, or maybe show your nephew a soccer anime I'd go with Inazuma Eleven, the story isn't that perfect but it is kinda fun and entertaining to watch if you ask me.
Inazuma Eleven
this one is so fucked up and so not soccer anymore that its genius and entertaining af, BUT the story is most of the time not good haha
Inazuma Eleven
this one is more a childish one, yet its decent, and don't watch the sequel if you want to you can, but I did not like it