I'm far from experienced with this on a drake as mine are girls, but here's what I learned.
Keep him separated so he doesn't get the urge to mate. His penis should have gone back in when that phase passed, but obviously, his didn't.
In addition to the sugar, Epsom salt baths, and antibiotic cream, get some generic Prep H. Put that on him a few times a day. You should see it start to shrink. When it does, start pushing it back in every time you apply the Prep H.
This could take a few days or even weeks to get his penis to stay in, but all of the above help.
The most important thing is to keep him separated from the hens so he doesn't get urges.