I was playing with an idea for a nephew with one arm, but I never finished it to definitely say it would be a good solution.
I picked up a foot forward type bike. a 24" coaster brake sun beach cruiser at first, then a smaller 6sp huffy venice. the venice is discontinued and would take some searching to find I think there are a few other smaller foot forward type bikes around (don't know how big he is). with a seat that has a backrest on it. that way he wouldn't need to support any weight with his hands leaning on the bar. the backrest also holds you in place when trying to accelerate or push up hills (I put them on my own semi recumbents so I know that part works pretty good). I worked on a couple other things pertaining to it having gears and handbrakes, but his working hand is more able. a coaster might be a better deal for his situation.