Here is how my mom convinces my dad to go:
"These grandchildren of ours are only young once. They are growing so fast, and we’ve been given the opportunity to be a part of something magical with them. "
My dad doesn’t love the parks either, but he loves being part of a family trip with all of us together. Maybe each of the kids could think of something they love to do with Grandpa and help get him on board by painting a picture of fun for him there. Like you said, it doesn’t have to be parks all day every day of your trip.
Has he seen the video for POR on allears.net? Has he been to POR before? That video (on youtube) could convince anyone to spend a week in WDW!
You can also tell him that you want one big family picture in front of the Castle for your photo album. He can send it in his Christmas cards this year.
Good luck with convincing him! I really hope he goes.