Well, I'm no occupational therapist, but I have been through what your student has, so I can tell you a few things that worked for me. (warning: brain damage may vary between individuals.)
1. Mirrors. The only way I'm able to tell if my fretting hand is the right postion is to look in mirror because I have little "inner sense" of my own body.
2. Positioning. I often have to use one hand to correct the postion of the other because I'm not able to move it accuratly enough on my own.
3. Sit down. I found out the hard way that it's practically impossible for me stand up and concentrate on my hands at the same time. Most people don't don't reallise just how much of our brains is devoted to balancing on only two legs because we do it unconsciously, but doing so consciously actually requires quite a bit of mental effort. Performing a complex task like playing a musical instrument while remaing upright is literally too exhausting for me! Let him stay in whatever posture he finds easiest, even if it isn't proper.
5. Talk. Ask him questions. Ask him about his stroke and his recovery. Asks him what makes things easy for him. Ask what makes things hard.
And Most Importantly
Listen!
There are so many things that the human brain takes care without us even realizing it that it's almost impossible for those of us who have lost these skills top describe it. What's even worse is to try to describe it and then be accused of making things up. If he says something sounds strange, like his head won't talk to hand, ask him to explain, and remember that there is no such thing as normal. "Normal" is simply a word people use for things that get taken for granted.
These are not rules. This is not advice. It's merely a few observations from a different point of view, to be interpreted however you see fit. Feel free to make things up. Remember, you're only his music teacher; he's the one with the difficult task of learning "how."
There's also a some books on the subject on neurological damage, check with your library.
There's also a good site about strokes
here and
another site that's mainly about autism but also several good resources about motor impairments.
Edit: I also forgot mention that also recieved very little therapy; in fact, it was not until I was 17 that the figured out I even had a stroke. I also have autism, which tends to have it's own motor issues, that was also diagnosed at 17. Therefore, I have no way of knowing what was caused by the stroke and what was already present. Feel free to PM if you have any questions.
Edit #2
Found good exercise.
Your student sounds like me! 
'cept my stroke was a long time ago, so I've mostly learned to (mostly) deal with it on my own.
