I've had home offices in that size space several times. Here's what worked for me, and might work for you guys:
1. Go vertical in that current storage wall like B-J said, but also not wall-mounted for the reason you noted. Instead, get a deep, stable, sturdy, full-height shelving system from Costco (or wherever), everything goes up from there, make one of the middle deep shelves the desk, feet landing on bottom shelf or floor. If he prefers a specific keyboard height, he can put in a simple sliding keyboard tray -its mount attaches to the underside of a middle shelf, rolls in and out as desired.
OR,
2. Replace dining table with big, table-style desk. Use that for work and eating. Can add slide-under keyboard tray to that, too, if it doesn't come with one. This works better with a laptop, because who wants drinks near a computer? Not me, man.
OR,
3. Put conventional desk where dining table is, pop out folding table in room's center for meals.
OR,
4. Put conventional desk where he's sitting now, move the other stuff into stackable storage crates from Staples (each facing upward or outward).