It won't just cost the £250 ish for the console itself, you'll then need to fork out another £50 for him to be able to play online, which will have to be re spent every 12 months. Unless you go for a 1 or 3 month membership, but that works out as more expensive per month, and lets face it, he won't want to stop playing in a month or 3 months.
The game itself i actually find really boring, as said theres no actual gore, but the point of the game is to kill all enemy players, so just exploring isn't gonna happen, as people WILL be trying to kill him, even if he doesn't play that way, they aren't just gonna let him run around exploring!
You can prevent in game purchases by simply not linking a payment method to the account. Likewise you can use parental settings to prevent him from talking to/being messaged by other players, which unfortunately can be very nasty. I'm a grown adult and i've been reduced to tears before by the sheer awfulness some think it's ok to message to another player!
Honestly i think it was a very unfortunate thing you bought an outdated console so recently, as all games going forward are unlikely to be for the console he currently has, but for the next generation Ps4's and Xbox ones, so you will be having this issue a LOT from now on.
As for the "all my friends have it so it's not fair!" argument, you cannot control what their parents allow, nor are their children the same as your child developmentally, personality wise etc. Yes, he will feel excluded, and he could well get left out unintentionally as they will be talking about the game and of course, if they want to play something he doesn't have they're not obliged to not play it and play something he does have. It is a shit situation to be in, and the reason i am SO glad that all this online gaming wasn't a thing when i was a kid. I feel for you, and every other parent for how technology ruled life is these days.