Sorry to here about your dad, Hope he recovers.

I would think step through design mow with large deck would work. He could step through using his left leg to run the pedal drive system on hydrostat models (automatic's) They also have elect. clutches to turn the deck on and off by pulling switch. John Deere X300 series or x500 series mowers may work. Or other manufactures with step through design.

He can go to JD dealer and see what may work for him and test drive them. JD's 300 series start out $4000. and X500 series start around $6000. with 500 hr bumper to bumper warrantee. Most people are satisfied with JD dealers due to their commitment to service.

With his right side weak He will just have to see if he can operate it side saddle with his left side operating the pedal drive. You will have to as if they have any special left handed/legged modified lawn tractors. I dough it, but won't hurt to as Dealer or goggle?

There are some Commercial stand up models you could research see it he could operate them with only left side functioning. Might try to google them also and find dealer to check them out...

Thank you rcmoser. We appreciate these ideas.