Honestly, I'd rent your stepdad a
scooter. So soon after surgery, I can't imagine him doing any Disney park for even one day comfortably. We had to rent one for my fil a few years ago and if we hadn't the whole trip would have been a nightmare for him and us.
We used Randy's Mobility, they deliver to hotels on and offsite, its cheaper than the ones in the parks and also they pick it up when you are done. It was a lifesaver for my fil and let us all enjoy the days and nights we had paid for without making him physcially miserable and exhausted. Or making us feel like we needed to scale back which also would have made him miserable. He always had a place to sit for a parade or fireworks, he had a great time and so many other people use them. Disney is not an easy vacation if you have any mobility issues.
For sure AK and Epcot have a ton of walking. But there is so much to do at both for all ages. My inlaws and parents all loved both of those parks and so do my kids. As for DHS, it does have two thrill rides but it also has Fantasmic, some great live shows, Toy Story Mania and its a smaller park as far as walking. AK also has two incredible live shows, the safari, Flights of Wonder which is one of those smaller shows Disney does so well. Just so much to do. If you have a nature loving child, don't skip AK.
Get the scooter for your stepdad. The best thing you can do for him is maximize his trip and ability to enjoy the grandkids doing all the things Disney has to offer at all four parks.
Check out the Disablities Board for all sorts of links on scooter rentals. That's where I found Randy's and we loved using them.