I'm a big gamer. Is he wanting to play online more than offline? At his age it'll most likely be online with his mates than offline I would imagine which does change things with content.
CoD isn't that bad it's just a shoot em up, there's minimal gore factor. It's fun to play online against others and there A LOT of kids his age on there.
RDR2 story mode is unsuitable, there's rape, murders, torture, druggings etc etc (amazing game though). Online is fine, there's none of that. You do hunting and collecting missions, bounty hunting etc. You can come up against other players but there's no too many online now, you can seek out opposition play matches for quick draw challenges etc if you want.
Halo is good and that's coming out soon. Again a shoot em up but against aliens. Very tame for the most part. I don't know if it's online, I've never played it online anyway.
You're going to struggle with a combat game that's pegi 16 that's any good imo. A bit of blood or bad language bumps it up to 18 pretty fast.
Fortnite is okay but a bit kiddish, it's free though.
Gta is fun online, it's just kids shooting eachother with bazookas, planes and tanks nowadays. No gore.
Story mode is sketchy in places but not too bad, it's mostly nicking cars and robberies but it depends on what your violence limit is. There are quite a few sex jokes and references, usually comedic and taking the mick out of someone. There is prostitution but you really have to seek that out.