Consider some of the comfort bikes that are available at your local bike shop. You don't have to drop big time money either. This type of bike is designed and marketed to people your dad's age and situation. (new rider)
The bike will be very upright and comfortable and also have a suspended seat post. My name is Ranger because i bought a Schwinn Ranger from Target about 19 months ago. I took it back to Target and got a refund because i could tell i needed a better bike. I then went to the LBS and bought a Trek 4300 for a couple hundred $ more. 9000 miles later, i am having a blast and am in great shape. I now have a new hobby that i am obviously obsessed with.
If I would have kept the bike from Target i would have likely quit by now. It was very suspect in terms of quality. It also was too small for me. Those bikes come in one size, whereas you get multiple sizes at the LBS.
A couple hundred bucks might be chump change if your dad takes to riding and reaps the many rewards it can offer.