I don't want to scare you, and it may not be what is wrong with your rooster, but Mareks disease can cause a droopy wing as well as problems walking--stumbling, paralysis in a leg, curled toes. It's a strange disease with differing symptoms in different birds. Reading a little information about it may help you rule it out, and of course if he is vaccinated the chances would be less. Here is a link to read:
http://extension.unh.edu/resources/files/Resource000791_Rep813.pdf
https://www.google.com/#q=marek's+disease