You might want to research "Hand Crank Trikes" on the internet. Many companies manufacture these bikes and there are many styles to choose from. Once you decide on a couple of possibilities you can generally find dealers listed on the manufacturer's web sites.

I rode with a gent last summer who was piloting a Hand Crank racing trike...well I didn't really ride with him. We started and ended at the same place, but got there much quicker than I did!

Good luck to you and your Dad. I hope you find a bike that he falls in love with.