I am quadriplegic and also a computer programmer, roboticist, compulsive Raspberry Pi collector and tinker so maybe I can offer some assistance. I do these things with the use of a reflective dot on my glasses, some voice dictation software and a button under my right index finger.
If you young acquaintance has the ability to slightly move his head, move one of his fingers or even any of his appendages reliably and can speak then setting him up with something where he can code is totally doable. And to be honest if he can't do all of those things it's still doable, but coding will just be a little bit more time consuming.
One slight word of advice though, in your original post you say "so he can feel useful again" and I totally get where this sentiment comes from and I'm completely sure that had nothing but good intentions when you said it. But I did want to say that rooting your young friends sense of worth in what he is able to physically accomplish might not be a great thing for a quadriplegic, as we are not really noted for our physical prowess!
Maybe if you couch it in terms of an intellectual challenge he might not feel so bad if this process doesn't work. I mean, there's no reason it shouldn't work, but you know, being realistic yay!
All that being said, either reply here or hit me up at: https://robotsandcake.org
and I will be glad to offer some assistance.