Take him to the vets right away. Keep it moist with KY jelly until you get there. You can also soak it in cold sugar water.
This is how I handle prolapse: Soak in cold water with as much sugar dissolved in the water as you can get. Soak 5 minutes and take out for 5 minutes and apply KY Jelly, then soak 5 more minutes and then apply KY. Continue to do this until either he pulls it back in or you get him to the vet. If he doesn't pull it back in on his own he will probably need surgery. You can try to help him get it back in after you have soaked it for awhile and the vet will try to get it back in. If it's been out for awhile the chances of him or the vet getting it back in are slim and he'll probably need surgery. I can only imagine how painful it must be. Do you have a good Cham vet? If not where do you live and I might be able to help you find one.