If the protrusion is located exactly where his vent is, this is likely a phallus prolapse. A phallus is a simple term for a Drake penis. There are a few predisposing factors for a drake to develop such as condition, but trauma to the area seems to be a predominant cause. If the area is left, feces will begin to accumulate in his GI tract. In addition, the phallus may begin to swell, become inflamed, or turn necrotic.
My first suggestion would be to set him in a warm, humid area inside your house. The warmth/humidity will help loosen the surrounding tissue and enable him to contract the phallus in easier. If you have an antibacterial ointment such as Neosporin, I would apply a copious coating of that onto the tissue. Preparation H, honey, or DMSO gel would also work. High emphasis should be put on keeping him hydrated as well.
After some time, attempt to gently work the phallus back into place. If you are unable to properly get the phallus inside, he will need to see a veterinarian experienced with poultry.