I think the biggest problem I'm seeing with a lot of these suggestions is, quite simply, pumping. Even if you attached a Big Blast to a bracket out in front of him, pumping it could be awkward. If you don't want his arm getting insanely tired from having to pump all the time without the added benefit of a second arm to do some of the work, I think your best bet would be a 3B.
Could we get a pic of the powerchair he has? Better yet, can you tell me what the brand name and model number is?
This is based on a non-powered wheelchair, but hear me out:
Attach the 3B's front handle to a small frame, say, made from copper pipe and couplers, that extends out directly in front of him. Throw a speedloader-style staggered barrel on the thing, so it's easy to reload, and he's instantly able to prime the gun with one hand, aim and fire it with one hand, and all he has to do is let go of the back handle and it just rests in his lap, pointing the barrel up (and bringing it closer to him) for easy reloading/rotating of the barrel (again, with the same hand). The gun's already accurate, consistent, and a nice performer as far as range is concerned. The added benefit of having a frame to steady his shots would just make things even more fun for him.
If you're having trouble visualizing this, let's take a look at things from am MSPaint perspective:
It could be made for under 10 bucks, and then strapped to the armrests relatively easily...say with a few zipties. Note that I didn't draw in his other hand, which would, of course, be controlling the wheelchair.
If you're interested, but lacking materials, time, and/or drive to pull off something like this, I'm pretty thoroughly intrigued by the idea, and I've got a 3b lying around, so if you'd like to PM me some measurements of the wheelchair (specifically, the distance from just outside one armrest to the other) I could be persuaded to build one for you, work out the kinks, set up the 3B to shoot just about whatever you want, and ship it, on me.
This post has been edited by Falcon: 21 April 2010 - 10:01 AM