Its the yolk sac that hasnt fully been drawn up into the little guys belly by the looks of the pic. _ but hard to tell.. Is that what it looks like???
If it is - it will reduce in size soon. He might be left with a small dried up bit the protrude and will eventually fall off. I have had this a few times with my ducklings. Most of them have survived. I would wait just a little while to put some antibiotic or antiseptic on it. I wont suggest anything in particular.... as you have different products to me.. Im sure someone else could recommend what to use.
In the mean time...keep him warm and quiet, and come on back and let us know if the bulge isnt smaller in an hour or two - or the morning.. It may be a sign of something else wrong as well. If you have some vitamins - a few drops of water with vitamins may help him gain his strength and perk up a bit - dosage would depend on what you might have available.