It does look like fowl pox, a virus spread by mosquitoes. The dry type lasts about 3 weeks, but since he is really sick, I would look inside his beak and throat for any yellow spots or tissue that might be signs of wet pox. The wet type makes them very sick, not eat or drink well, and he may need some help getting enough to eat and drink. Secondary infections can also be a problem with pox. I would mix up a small bowl of chicken feed, egg, a tsp of plain yogurt, and lots of water. Giving some poultry vitamins would probably help.