Even if you get it back in take it to the vet. If you don't it will just prolapse again. It dosent just happen so you need to find the underlying cause and fix it. It could be kidney stones from chronic dehydration or a worm infestation, anything that's causing it to strain when pooping/pee
Keep him moist and apply sugar paste. You can also use some KY jelly to keep it wet. I found that my tort had twisted his willy and couldn't get it back in, i had to manually twist it and then i set him on a coffee can and let it dangle for a minute while I prepared the sugar paste and he managed to get it most of the way in on his own.
Also, remove his substrate and keep him on moist paper towels for now.
I really have a mixed bag of feelings reading this thread, anyone else ?
Educative : i didn't know that, lots to learn
First Aid Solution : the sugar paste
Funny : the "twisted willy" and "dangling" references
Painful : the "twisted willy" reference, coincidentally
Thankful : I'm not a male tort...