So if a game has windowed mode it's possible for him to reach most areas of the screen?
Yes, that's how he was able to play both FTL and Into the Breech. FLT required him to use the onscreen keyboard to press the spacebar to pause and unpause the game and luckily that was one such game that took it well some games just don't recognize the onscreen keyboard. Then again FTL was some years ago now so I don't know if he'd still be up to pausing and unpausing nearly as much. Every year he loses a little more of what he can do and things that used to be easy will eventually become difficult and then impossible.
Is he able to click and hold or just click once?
Yes, things that are difficult for him are right clicking, scrolling the mouse wheel, or anything that require a rapid double click. Something like clicking both left and right mouse buttons would be impossible since he requires both his index and his middle finger to click, we place both his fingers over the button and he uses the weight of them to aid in the clicking and it only works if we get his hand, wrist and arm just right.
Although one caveat at the moment is that you need to move between windows to copy/paste level codes into the game, so I wonder if that's already too inconvenientI think that would be fine, as long as drag and drop works. He can't right click -> copy -> right click -> paste but highlighting a block of text or grabbing a icon and dragging it isn't a problem for him at the moment. I've also seen him use windows onscreen keyboard to ctrl+v text as well.