My guess was due to the size of the hole, I don't think a fox would make such a large hole. Maybe you can ask your son how big the animal he saw was. Foxes are pretty small compared to coyotes.
First off, chicken wire is designed to keep chickens in, but not keep predators out. Hardware cloth no larger than 1/2" should surround the entire coop & run. No gaps anywhere larger than 1/2".
Obviously, I can't be sure, but it sounds like a coyote took your pullets. If it was a coyote, it...