Been to Colorado 3x's for Elk. All w/ a bow. Its mostly motorized vehicle restricted. Forever wild wilderness area. You can only drive motorized vehicles, including atv's to the trail heads. From there its strictly all on foot, or horse back. I'd highly recommend horses if plan to hunt the flattops. Your atv's would only get you as far as your truck could. Its a huge area with different park areas amongst it. Stump park, Derby ridge park, Elk Park, Bear park to name a few. The Elk can be anyplace in the flatops, and you'll have to cover alot of ground, I mean miles sometimes to find them. I was riding on average 2-6hrs a day on horseback to find Elk. No way I could've walked those distances. So it may not be what your looking for. Even if you decide to spike camp it in a certain park area of the Flat tops you'd need horses to get your gear in & out.
Otherwise if atv's are your only means to get your father around. I'd look into private land.