I would say glasses and lowering the FOV. I see a lot of people recommending a smaller monitor, but that only helps his neck, not his eyesight. In fact, that would make it more difficult to see.

Unfortunately, there's not really any options in the settings that can help him much, other than FOV. Most settings you might consider aren't implemented as they can create unfair advantages for players who don't need the accessibility settings.

Personally, I would recommend trying out other shooters. I'm not sure what system you're playing on, whether it's PC or console, but I would recommend Valorant on PC. It's inherently a completely different game than CoD, but the reason I recommend it is because enemies are highlighted red, making it much easier to see. There's other games that have that feature also that you can check out. But I feel like finding a game that highlights enemies will go the farthest in helping him see better.

Shooters with super high realism can be tough for anyones eyesight. I'm young, and my eyesight is great, and I will still have trouble seeing people on occasion depending on the light and contrast and such. Games with a cartoony art style will always be easier on the eyes.