Season bison liberally with salt and pepper on all sides. Light one-half chimney full of charcoal. When all the charcoal is lit and covered with gray ash, spread evenly over one side of coal grate. Alternatively, set one set of burners on a gas grill to medium-low heat. Set cooking grate in place, cover grill, and allow to preheat for 5 minutes. Clean and oil the grilling grate.
Place bison over cooler side of grill with fat cap facing up. Cover, leaving all vents half open, with the lid vents positioned over the meat. Cook, turning and rotating occasionally, until bison registers 115°F on an instant read thermometer, about 2 hours. If using a coal grill, add 12 coals on top of the old coals 1 hour into cooking.
Transfer bison to a large cutting board. If using coal, add ½ chimney of fresh coal to embers and allow them to ignite without covering. If using gas, set two sets of burners to highest heat, cover, and allow grill to preheat for 10 minutes.
Return bison to hot side of grill and cook, turning occasionally, until well browned on all sides, about 8 minutes total. Internal temperature should have risen to about 125 to 130°F (medium-rare). Let bison rest 5 minutes, carve, and serve.