It's a BS excuse. Being a parent myself, I wouldn't cater to his trying to own multiple brands of consoles for a reason as flimsy as his..
Just ask him what makes him think he's better at the game then on a PS.
I'll bet my ass that he cant come up with a coherent statement within 2 sentences. Not an adult one, but one that would make sense.
Ergo, he wants an Xbox just because. Either that, or a disgruntled friend/ classmate owns an old Xbox One and says he's better at the game, which is a laod of BS.
Yes he's a kid. Yes he will be disappointed if he doesnt get what he wants, but he'll have another learning moment of coping with disappointment. That's growing up for ya...
Dont give him this. Since he can play on a Ps4 Pro or a PS5. Be stern.
Or give him a choice. In comes Xbox One? Out goes the PS5 (yes the best console needs to go). Sit back and watch him do a 180.