Here's something we need to know...is he going to get better, in regards to finger control? Or is this essentially permanent?
I realize the goal is to get back to playing guitar, but in his current state, maybe that shouldn't be the first goal. Strumming a guitar is 100 times easier than the fingerings; if he's having trouble strumming, then the fingerings are far off. He needs something else to do to start making those connections to his fingers again. Can he play a keyboard (i.e. piano)? Can he type on a computer keyboard? What other activities can you challenge him to achieve, that will get him back on the road to playing the guitar?
I'm no physiotherapist, and barely a guitar player; but it seems there are some intermediate steps we have to climb here. If he can barely strum, I have to assume he needs a lot of other PT/OT as well.
Keep in mind, guitar playing isn't all that easy to begin with, even if we have complete control of our fingers. If he truly isn't going to get better, then some of the alternative devices/styles/instruments mentioned in other answers may be the key.