Grace -
Try to start him out with some gentle seated movements first, and see how he takes to this. Maybe you could have him sit in a chair and breathe in as he raises one or both arms up to the sky, and exhale as he lowers them. Or he could sit in the chair and extend one or both of his legs on an an inhalation too. These simple types of exercises performed with the breath can be very powerful and give much needed gentle movement to major joints and to the spine. If he doesn’t get dizzy, tired, or have any trouble with these postures, then you can add some gentle neck movements, or even perhaps move on to more challenging movements, but see how he fares with this activity first.
Outside of this, it is impossible to guess what is in his capacity. It would be best to have a therapeutic yoga teacher evaluate him and offer suggestions based on the state of his body and his breath as he is now.