An 18 gauge needle connected to a syringe would be good to deflate a leaking air sac. I have used a tiny 22 gauge on a chick before which was all I had, and it worked well. Do you have another rooster who might have bumped him?
Does it feel soft like a hematoma (blood under the skin,) air, or hard like a mass? I am going to tag @azygous and @Wyorp Rock who also might have a look.