Neosporin (no pain reliever) and push the entire thing back inside is the first option for treating prolapse.
He has to be kept away from the girls for a prolonged period of time to allow it to heal and stay in. Breeding can cause it to get worse and not correct.
If it has been out (exposed) and is now turning necrotic it is time to take VERY sharp scissors (we use medical paramedic sets here) and cute behind where the dead tissue is, leaving behind only healthy tissue. Like a rubber band pulled tight, when clipped it usually recoils back inside quickly.
We had a muscovy drake we had to cut this spring (May). He is back to full health this fall and breeding very well again now too. He was kept separate from the girls for 2 months, then one girl introduced so we could make sure he is ok to rejoin the group.