Sadly, being big is expensive.
Of the guns you listed, the only one that suits really big hands would be the Glock--but you'd have to go with either the 20 (10mm) or 21 (.45 ACP), neither of which is cheap to shoot without adding a .22 conversion. My G21 is the most comfortable factory grip I've ever found.
I've got a Walther P99 with interchangeable backstraps, and that arrangement is not all it's cracked up to be. Even with the largest backstrap, it's too small--the grip is just too short overall, and the fat part of the largest backstrap winds up in the wrong part of your hand. I haven't tried the M&P, but I suspect it would be much the same.
I manage OK with a 686 using the factory Hogue stocks, but it gets uncomfortable when the recoil gets stiff. There's just not enough to hold onto.
Eagle and Ajax both make decent aftermarket grips for revolvers, including the K & L frames. They're on the small side for me (XXL glove), but they may do okay for your boy. If they seem too small (depending on your locale, they can often be checked out at gun shows), Herrett's makes grips to measure. They made a set of Jordan Troopers for my Redhawk that are just massive. Unfortunately, I'm not wild about the Jordan Trooper design. Nill Grips makes some grips in an "extra large" size that I haven't had a chance to try out yet. They're not cheap, though--around $140 each (which is one reason why I haven't tried them yet).
If you're looking at a .22 semiauto, give some thought to the Browning Buckmark. It's got a longer and deeper grip than the MkIII, and with some aftermarket grips (either Eagle or Ajax makes a nice set of finger-groove grips for Buckmarks), they'd probably fit him quite well. I've got one myself, and while it could be a little bigger I can shoot well enough with it--even with the factory grips.
