The hemipenes are designed to lock inside the female until the male is done. Being dragged is normal, but not necessarily preferable.
If the hemipene is still extended a few minutes after mating, I wouldn't worry too much. Just remove him to a container with no substrate so he doesn't get wood chips or other muck on it. Wash it with cold water as Frank suggested.
If it stays everted for more than an hour, you might start thinking about pushing it back in as Frank suggested. You don't want it to dry out, and you don't want it to get dirty or cut.
It should go down on its own, but if it doesn't, you need to either follow Frank's advice, or if you don't feel comfortable with that, take him to a vet.
