My lad (12) is getting really into anime. He's watched and re-watched Haikyu and is starting My Hero Academy. Are Attack on Titan and Full Metal Alchemist suitable for him? I'm okay with some mild swearing and some blood and guts.
Attack on Titan is way bloodier than My Hero Academia. It's fairly brutal. I'd suggest testing the first episode yourself first to see if it's a level of brutality that you're comfortable with.
Full Metal Alchemist is a bit less bloody, but it has some fairly full on themes. Loss of parents, child/animal abuse, genocide etc. It's the sort of thing I think would make a deep impression on a 12 year old, but it might be helpful to watch it with them so you can help him to process some of the themes - depending on how sensitive they are to these things.
I reckon Demon Slayer is actually a really good anime for a 12 year old. It'd be challenging enough in that there are some of those themes, and certainly has enough blood (without being quite the level of Attack on Titan). It's currently only 1 season and a movie with a season to come, so it's not a massive commitment.
Death Note could also be an interesting one that would still be interesting to him until he's a few years older as well.
Alternatively if he wants to watch something that just goes on and on and on, there's always your Hunter X Hunter, One Piece etc.