I'll also recommend the Flying D. I shot a bison there back around 2000, and they made it a nice experience. The ranch is something like 100,000 acres, and has very few internal fences. You can stay in Bozeman, or any of the other nearby towns, and you'll meet your guide that morning.
It is generally just a one day hunt, and your dad will have a very good time. If you tell your guide about your father's situation ahead of time, I'd expect they'll go out of their way to show him a good time. I was looking for a "non-trophy" bull, which is less expensive than a "trophy" bull, and the only difference is a non-trophy bull is usually very old with broomed horn tips, while a trophy bull has perfect or near perfect tips so that he'll score better. A non-trophy bull is quite a bit less expensive and still looks great on the fireplace. We rode around looking at lots of bison before getting serious about finding the one they wanted me to shoot. They had a particular bull they wanted me to take, and he was hanging out with a trophy bull out toward the northwest part of hte ranch. After looking at a lot of bison, we wandered over that way late in the morning and found them together, feeding along a hillside. They're not very spooky, and you can approach them pretty easily. We got up to about 50-60 yards and I shot the bull. They're tough! You don't have to use a big rifle, a .30-06 would be great, but you should use a very well constructed bullet capable of penetrating. I'd recommend a nosler partition, barnes triple shock or something along those lines. I shot mine with a .338 and 225 gr. trophy bonded bearclaw and it worked well.
The "non" trophy bull I took ended up being a 12 or 13 year old bull, the guide wasn't completely sure other than that he was among the first releases they did on the ranch. I don't remember how much the weight was at the processor, but do remember them telling me the bull likely weighed close to 2.100 pounds live. I don't doubt it, either; as I've shot 6 cape buffalo and this bison was a lot bigger than any cape buff I've shot.
I was guided by "JT", James Thompson, who I believe is a professor at Montana State when he's not out playing on the Flying D. He was a great host and made our day very nice. If he's still guiding for Rob Arnaud, I'd highly recommend him. Bison are fantastic eating, and the head makes a great mount. The Flying D crew take care of everything for you, including taking the bison to the processor and head to a local taxidermist. You get all the meat, the head and hide, and the cost is lower than many other places, while allowing you to take an older bison than most other ranches can provide. It's a good value and fun experience.