First of all, I’m sorry to hear about your Dad’s health. What a difficult thing to deal with while you are planning your wedding.
My Mom does not have a terminal illness, but has Progressive MS, so I can related to some of your concerns. I also worried about this leading up to our wedding. Like @Vsmart: suggested, try and keep him as involved as possible leading up to the wedding (as much as any Dad would allow)! I know this really helped my Mom.
Will he be there when you are getting ready? Maybe you could take a few moments during that time (or at another time during the day), and give him some sort of special gift/letter so that he knows how much he means to you.
Before the wedding, I asked one of my bridesmaids to keep an eye on my Mom, and to make sure she always had a snack/drink/whatever she needed (Maybe you could do the same, or ask a family member to). Since pretty much everyone there knew my Mom, she was well taken care of, my bridesmaid didn’t need to do anythig for her! I also noticed that on the wedding day, most people were coming to my Mom and chatting with her (instead of her having to go to them). Since she had a walker/forearm crutches it was obvious to anyone that doesn’t know her that she has mobility issues, and they made the effort to reach out to her. Honeslty, she was exhausted, and in so much pain, that she had no desire to dance and run around!
You Dad will understand that you can’t be by his side the entire night, and I bet he wouldn’t want you to be. Take a few moments throughout the night to give him a hug, a smile and a wave, or to check in to say a quick hello. Make sure someone is assigned to assit him, and that he is front row for all of the special moments of the day (ie. cake cutting, first dance, speeches.)
ETA: Like mentioned above, we had a sildeshow, and included were lots of photos of my Mom and I together. This made her feel so special, and was one of the favourite parts of the night for her!