I've dealt with this only one time and it was with my Nosy Chia about a year ago. Except with Chia....it was both Hemipenes.
The first thing you want to do is keep it as moist as possible....as much as possible. Baths may not be the best choice, as sitting in a tub of water is not exactly a natural occurance for chameleons. It adds alot of stress in an already extremely stressful situation. It may be good for iguanas and other lizards/snakes, but chams I definately would discourage this practice.
Chia decided he'd have his problem on the weekend as well. I attempted myself to force them back in using a sterile heavily lubricated q-tip and the help of my wife. I got nowhere.....So I had no choice but to wait until monday. What I did in the mean time was use a piece of gause that was as wet as I could get it to wrap the area. Pretty much I custom made a chameleon diaper. Then I used an ace bandage to hold it on. I'm sure if you could figure out a way to keep the outside layer dry (a peice of plastic or something similar) you could also tape it on if need be. Monday came and I took him to the vet. My vet is fairly good with reptiles in general....but when it comes to chams he's not exactly an expert. Not many vets are. Pretty much whenever I go with a problem, I tell him what I think it is....and he either agrees or disagrees and we work together to try and get to the root of the problem. But in the case of prolapses, most herp vets have seen their share of this problem in other reptiles. He knew exactly what to do, and he had me hold chia through the entire proceedure. Slowly, over a period of about 15 minutes he carefully and gently kept applying a little bit of preassure in all the right places and they ended up going back in......and staying in for good.
I could tell in the next couple of days he kind of favored the area...and it didn't look exactlly normal, but after about a week there was no sign that he had ever had a problem. I had to take him out of my breeding program for a while just to be safe, but since then he's bred to another female.....and her eggs appear to be quite healthy and on the way to a high hatch rate.
Hope this helps.....and good luck with him.