I was thinking same as Philly.
God forbid that something like this would happen to me, but I think I'd have been on the guitar already in a "ok, all bets off, how can I play it now?"
Are you saying in your original post that he has been playing, but just strumming?
I think I'm with you HTH - the thumb and 3rd/4th method would seem to be the most workable. But that's partly based on I can't get comfortable using finger picks myself anyway, so the concept of controlling an attachment that goes from the 1st joint seems completely off the map for me. If I had thumb and two fingers, and could balance/control the hand, I'd be looking at that.
EDIT: Just had a thought afterwards, if it's a few years down the line, he must be able to use his 3rd and 4th more proficiently than most of us anyway by now? Just with general day to day stuff like tying up shoe-laces or whatever? Or am I making daft assumptions here... not something most of us tend to think about (or even want to think about) most of the time :(