An employee was welding inside of a wing tank for about 30 - 40 minutes. He came out of the void at about 1500 hours feeling lethargic and weak and was assisted to the cool down area. The employee left the cool down area and went to the office where the decision was made to take him to the hospital. He was diagnosed with heat illness and treated with fluids. He was officially admitted to the hospital at about 2000 hours. The injured employee was wearing leathers, a welding shield, and gloves during welding.