An Xbox Adaptive Controller paired with the Logitech Adaptive kit can get him most of the way there. He would still need an analog stick replacement. You can find some great custom ones on etsy, like from SevenMileMountain
Mouth controllers are crazy expensive and usually sold out. https://www.quadstick.com/shop
Sadly there is no one boxed solution for it. And you'll probably end up spending over $250+ to get it all together. The disabled tax is brutal.