Flat tops is basically a wilderness area, so you have to park the ATV at the trailhead and walk (or ride a horse) into the wilderness. Sounds like The Gunnison area would be your better bet. Just remember that only the very lucky few will ever kill an animal within sight of a road, so plan on hoofing it and packing out your game. If you want an area your dad can hunt on, you might consider trying a tresspass private land hunt where you can do more "stand" type hunting. Public land usually means lots of moving around by foot. Otherwise you might consider renting a horse for you father and you walk. You can also use the horse as a pack animal. Lots of options!