you absolutely do not want to let that sit. as for painkillers, give him some very low-dose painkiller. you can crush the pill in half and mix it in with a little bit of water or food to give to him that he will eat right away. Advil has 200 mg which is way too much but if you have anything like baby aspirin that is around 80 mg crushing that in half would be okay for him, anything around 40mg wont do him harm. if you can, get him to a vet immediately, they will most likely fix him right up, but if you don't have any nearby vets.. you could try to do it yourself, although you could risk causing the rooster pain, not setting the bone right, making the bandages too tight, etc.. if you can, the vet is the best option. I had a pullet with a broken leg, broke it when it got caught in a fence trying to escape a fox, and i thought it was dislocated until i brought her to the vet and they said it was actually a fracture but it was too late to do much other than an attempted splint but it healed totally backwards and the poor thing could barley walk. it was too late for the splint to do any good.
where is it broken, what does it look like, how did he break it, how long ago, etc.? pictures?