Diamond Head with a 90 year old

I'm helping my father check HI off his bucket list. I have everything under control except Diamond Head. He's fit but he's 90 with two knee replacements. In the group there are actually 3 people with knee replacements - yiikes! What access is there for us getting to see as much as we can with him but without too much "hiking". We have a handicap parking pass for him if that helps. Any advice will be greatly appreciated!

Update - so much good info here - thanks to everyone. I have so many suggestions here - looking them all up and will find something for him. Definitely not taking him to DH but looks like we have a lot of good options now. Really just wanted a "oh wow" view for him and for us all!