How old is he, and what type of feed are you feeding him? It is possible that he has a vitamin deficiency. Or, he could be weak/tired from being picked on or because of the heat. Take him inside, give him some water with (if you have any) probiotics, vitamins, and electrolytes in it, and make sure he is eating. If you really think he's hot, mist him with water and/or put a fan in front of him.
Feel his legs for any swellings, heat, sores, or fractures. Feel his breastbone. Does it jut out a lot (in which case he's thin)? If so, worms could be a possibility. If so, you should worm him. I use a wormer known as the Worminator, which can be purchased from here:
http://www.twincitypoultrysupplies.com/store/index.php?main_page=product_info&products_id=584 It treats all worms, except tapeworms, has an egg withdrawal period of 24-48 hours, and is easily administered orally. The dosage for large-fowl is 2 drops, and for bantams it is 1 drop. Other wormer choices include Valbazen, SafeGuard, Ivermectin, or Wazine.