You'd probably have to replace his desk to make it fit, but Fully for example makes a lot of different sized desks. There's also standing desks converters that work with regular desks, wirecutter has done a bunch of reviews. Does he have access to any sort of 'ergonomic review' of his workspace through his employer? That might be another way of finding things that are contributing to his pain. When I got my standing desk I loved it in theory but not in practice, it took a while to get into the habit of standing regularly, I started with just committing to standing in meetings.
~ Dawn
Our little monkey (4/2011) & his early holiday present 12/12