It's called prolapse of the hemipenes and can be a serious problem. The hemipenes need to be replaced back in their pocket or they will dry out, become necrotic and fall off. I had one prolapse one hemi this year. I used a very concentrated sugar solution (actually powdered sugar, but hey, whatever works) to wick some of the water out of the hemi and get it to reduce in size until I could gently manually ease it back in place.
You may try the sugar solution tonight...fill the bottom of a deli with it and let him sit in the sticky mess for a half hour to reduce the pressure by drawing fluid out of the hemis. If you are lucky, they will get down to a size that they go back in by themselves. If not, you need to get that snake to the vet if you want any hope of saving him for a breeding animal.
The longer they are out, the worse the prognosis, so I'd give the soaking a try, but get him to a vet.
I gave my guy the year off. I'll try him next year.