Whisk onion powder, salt, smoked paprika, cumin, cayenne pepper, garlic powder, oregano, thyme, and brown sugar together in a small bowl until thoroughly combined.

Make 2 cuts in chicken skin, 1 on front and 1 on back; loosen skin without tearing by inserting your hand underneath skin. Rub chicken skin and cavity with vegetable oil; rub spice rub over chicken skin, inside cavity, and beneath loosened skin.

Preheat an outdoor grill for medium heat.

Place the beer can on the preheated grill; slide chicken onto beer can by the cavity. Grill until chicken is thoroughly browned, the juices run clear, and an instant-read thermometer inserted into the breast, near but not touching bone, reads at least 165 degrees F (75 degrees C), 1 to 1 ½ hours.