Hi,
I haven't got much personal experience with the hand cycler, but it is worth a try. Suggest to give him guidance first, i.e. hold his hand/ elbow in place while he does the cycling. Have you ever tried 4pt kneeling, so getting him on hands and knees - could try to sit back on feet (while you are standing over him and hold pelvis/ carer could hold hand in place if neccessary) and slowly shift weight forwards onto hands?
I don't want to go to deep into the possibilities of having a degenerative disease. That's completely hypothetical. Not seen as a consequence of the TBI, but rather a new/ different event. Don't go there, I'm nearly sorry to mention it ...

Cheers,
Fyzzio